UPDATE: Google have just published a blog article called Google Instant, behind the scenes. It's an interesting read, and obviously related to this question. You can read how they tackled the extra load (5-7X according to the article) on the server-side, for example. The answer below examines what happens on the client-side:
Examining with Firebug, Google is doing an Ajax GET request on every keypress:
I guess it's working the same way as the auto completion. However this time, it also returns the search results of the partially complete search phrase in JSON format.
Examining one of the JSON responses while typing "Stack Overflow":
We can see that the JSON response contains the content to construct the search results as we type.
The formatted JSON responses look something like this:
{
e: "j9iHTLXlLNmXOJLQ3cMO",
c: 1,
u: "http://www.google.com/search?hlx3denx26expIdsx3d17259,24472,24923,25260,25901,25907,26095,26446,26530x26sugexpx3dldymlsx26xhrx3dtx26qx3dStack%20Overflowx26cpx3d6x26pfx3dpx26sclientx3dpsyx26aqx3dfx26aqix3dg4g-o1x26aqlx3dx26oqx3dStack+x26gs_rfaix3dx26pbxx3d1x26fpx3df97fdf10596ae095x26tchx3d1x26echx3d1x26psix3dj9iHTO3xBo2CONvDzaEO12839712156911",
d: "x3clink relx3dprefetch hrefx3dx22http://stackoverflow.com/x22x3ex3cscriptx3eje.pa(_loc, x27rsox27, x27\x3c!--m--\x3e\x3clink rel\x3dprefetch href\x3d\x22http://stackoverflow.com/\x22\x3e\x3cli class\x3dg\x3e\x3ch3 class\x3d\x22r\x22\x3e\x3ca href\x3d\x22http://stackoverflow.com/\x22 class\x3dl onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNERidL9Hb6OvGW93_Y6MRj3aTdMVA\x27,\x27\x27,\x270CBYQFjAA\x27)\x22\x3e\x3cem\x3eStack Overflow\x3c/em\x3e\x3c/a\x3e\x3c/h3\x3e\x3cdiv class\x3d\x22s\x22\x3eA language-independent collaboratively edited question and answer site for programmers.\x3cbr\x3e\x3cspan class\x3df\x3e\x3ccite\x3e\x3cb\x3estackoverflow\x3c/b\x3e.com/\x3c/cite\x3e - \x3cspan class\x3dgl\x3e\x3ca href\x3d\x22http://webcache.googleusercontent.com/search?q\x3dcache:U1GC2GYOToIJ:stackoverflow.com/+Stack+Overflow\x26amp;cd\x3d1\x26amp;hl\x3den\x26amp;ct\x3dclnk\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNFfKMag7Tq8CMbbfu8Gcj_GjukTbA\x27,\x27\x27,\x270CBgQIDAA\x27)\x22\x3eCached\x3c/a\x3e - \x3ca href\x3d\x22/search?hl\x3den\x26amp;q\x3drelated:stackoverflow.com/+Stack+Overflow\x26amp;tbo\x3d1\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CBkQHzAA\x22\x3eSimilar\x3c/a\x3e\x3c/span\x3e\x3c/span\x3e\x3cbr\x3e\x3ctable class\x3dslk style\x3d\x22border-collapse:collapse;margin-top:4px\x22\x3e\x3ctr\x3e\x3ctd style\x3d\x22padding-left:14px;vertical-align:top\x22\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/questions\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNHmP78gEctJKvBrydP2c52F_FEjvA\x27,\x27\x27,\x270CBoQqwMoADAA\x27)\x22\x3eQuestions\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/questions/ask\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNGZF-qwWVTZOWPlr4vgSA7qB64LLQ\x27,\x27\x27,\x270CBsQqwMoATAA\x27)\x22\x3eAsk Question\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/users/135152/omg-ponies\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNE9zo6Qi_AM1bjmPGeMGfbnPi3niA\x27,\x27\x27,\x270CBwQqwMoAjAA\x27)\x22\x3eOMG Ponies\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://careers.stackoverflow.com/\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNEaqlBrfDcc1gdPZ6dgthff0s5WmA\x27,\x27\x27,\x270CB0QqwMoAzAA\x27)\x22\x3eCareers\x3c/a\x3e\x3c/div\x3e\x3ctd style\x3d\x22padding-left:14px;vertical-align:top\x22\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/about\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNEqgPttrXj3r4o3TZHX5WaWvFe1HQ\x27,\x27\x27,\x270CB4QqwMoBDAA\x27)\x22\x3eAbout\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/faq\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNF3X3eRc0RsxYynXZhhbyYkuKWZ5g\x27,\x27\x27,\x270CB8QqwMoBTAA\x27)\x22\x3eThe FAQ\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://blog.stackoverflow.com/\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNG7KphjK6RuC5cj-6U5jeuvipt5dg\x27,\x27\x27,\x270CCAQqwMoBjAA\x27)\x22\x3eBlog\x3c/a\x3e\x3c/div\x3e\x3cdiv class\x3dsld\x3e\x3ca class\x3dsla href\x3d\x22http://stackoverflow.com/users\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x271\x27,\x27AFQjCNFfN_wcGm4HE5XpDxvcH4bIrkv2dw\x27,\x27\x27,\x270CCEQqwMoBzAA\x27)\x22\x3eUsers\x3c/a\x3e\x3c/div\x3e\x3ctr\x3e\x3ctd colspan\x3d2 style\x3d\x22padding-left:14px;vertical-align:top\x22\x3e\x3cdiv style\x3d\x22padding-top:6px\x22\x3e\x3ca class\x3dfl href\x3d\x22/search?hl\x3den\x26amp;q\x3d+site:stackoverflow.com+Stack+Overflow\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CCIQrAM\x22\x3eMore results from stackoverflow.com\x26nbsp;\x26raquo;\x3c/a\x3e\x3c/div\x3e\x3c/table\x3e\x3c/div\x3e\x3c!--n--\x3e\x3c!--m--\x3e\x3cli class\x3dg\x3e\x3ch3 class\x3d\x22r\x22\x3e\x3ca href\x3d\x22http://stackoverflow.com/questions\x22 class\x3dl onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x272\x27,\x27AFQjCNHmP78gEctJKvBrydP2c52F_FEjvA\x27,\x27\x27,\x270CCUQFjAB\x27)\x22\x3eHottest Questions - \x3cem\x3eStack Overflow\x3c/em\x3e\x3c/a\x3e\x3c/h3\x3e\x3cdiv class\x3d\x22s\x22\x3eHello \x3cem\x3eStack Overflow\x3c/em\x3e! I\x26#39;m working with someone else\x26#39;s PHP function that works fine as long as I pass it at least three arguments. If I pass it two argument, \x3cb\x3e...\x3c/b\x3e\x3cbr\x3e\x3cspan class\x3df\x3e\x3ccite\x3e\x3cb\x3estackoverflow\x3c/b\x3e.com/questions\x3c/cite\x3e - \x3cspan class\x3dgl\x3e\x3ca href\x3d\x22http://webcache.googleusercontent.com/search?q\x3dcache:6S_0sErDKfQJ:stackoverflow.com/questions+Stack+Overflow\x26amp;cd\x3d2\x26amp;hl\x3den\x26amp;ct\x3dclnk\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x272\x27,\x27AFQjCNH7WHzefYlnS05ln4j6rzfE3byDKg\x27,\x27\x27,\x270CCcQIDAB\x27)\x22\x3eCached\x3c/a\x3e - \x3ca href\x3d\x22/search?hl\x3den\x26amp;q\x3drelated:stackoverflow.com/questions+Stack+Overflow\x26amp;tbo\x3d1\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CCgQHzAB\x22\x3eSimilar\x3c/a\x3e\x3c/span\x3e\x3c/span\x3e\x3c/div\x3e\x3c!--n--\x3e\x3c!--m--\x3e\x3cli class\x3dg\x3e\x3ch3 class\x3d\x22r\x22\x3e\x3ca href\x3d\x22http://en.wikipedia.org/wiki/Stack_overflow\x22 class\x3dl onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x273\x27,\x27AFQjCNEAxaeWmWtD7cBcmZ5QBMsTRNbnCw\x27,\x27\x27,\x270CCkQFjAC\x27)\x22\x3e\x3cem\x3eStack overflow\x3c/em\x3e - Wikipedia, the free encyclopedia\x3c/a\x3e\x3c/h3\x3e\x3cdiv class\x3d\x22s\x22\x3eIn software, a \x3cem\x3estack overflow\x3c/em\x3e occurs when too much memory is used on the call stack. The call stack contains a limited amount of memory, often determined at \x3cb\x3e...\x3c/b\x3e\x3cbr\x3e\x3cspan class\x3df\x3e\x3ccite\x3een.wikipedia.org/wiki/\x3cb\x3eStack\x3c/b\x3e_\x3cb\x3eoverflow\x3c/b\x3e\x3c/cite\x3e - \x3cspan class\x3dgl\x3e\x3ca href\x3d\x22http://webcache.googleusercontent.com/search?q\x3dcache:mWu8b0BQAmwJ:en.wikipedia.org/wiki/Stack_overflow+Stack+Overflow\x26amp;cd\x3d3\x26amp;hl\x3den\x26amp;ct\x3dclnk\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x273\x27,\x27AFQjCNFG_5ndK-KmWJy6s3pOsi8lsxqEZg\x27,\x27\x27,\x270CCsQIDAC\x27)\x22\x3eCached\x3c/a\x3e - \x3ca href\x3d\x22/search?hl\x3den\x26amp;q\x3drelated:en.wikipedia.org/wiki/Stack_overflow+Stack+Overflow\x26amp;tbo\x3d1\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CCwQHzAC\x22\x3eSimilar\x3c/a\x3e\x3c/span\x3e\x3c/span\x3e\x3c/div\x3e\x3c!--n--\x3e\x3c!--m--\x3e\x3cli class\x3dg\x3e\x3ch3 class\x3d\x22r\x22\x3e\x3ca href\x3d\x22http://blog.stackoverflow.com/\x22 class\x3dl onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x274\x27,\x27AFQjCNG7KphjK6RuC5cj-6U5jeuvipt5dg\x27,\x27\x27,\x270CC0QFjAD\x27)\x22\x3eBlog – \x3cem\x3eStack Overflow\x3c/em\x3e\x3c/a\x3e\x3c/h3\x3e\x3cdiv class\x3d\x22s\x22\x3e6 Sep 2010 \x3cb\x3e...\x3c/b\x3e The latest version of the \x3cem\x3eStack Overflow\x3c/em\x3e Trilogy Creative Commons Data Dump is now available. This reflects all public data in … \x3cb\x3e...\x3c/b\x3e\x3cbr\x3e\x3cspan class\x3df\x3e\x3ccite\x3eblog.\x3cb\x3estackoverflow\x3c/b\x3e.com/\x3c/cite\x3e - \x3cspan class\x3dgl\x3e\x3ca href\x3d\x22http://webcache.googleusercontent.com/search?q\x3dcache:iqtvg9Ge1c0J:blog.stackoverflow.com/+Stack+Overflow\x26amp;cd\x3d4\x26amp;hl\x3den\x26amp;ct\x3dclnk\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x274\x27,\x27AFQjCNFX2P2-RTCs_GaR6NgSw30p007UEA\x27,\x27\x27,\x270CC8QIDAD\x27)\x22\x3eCached\x3c/a\x3e - \x3ca href\x3d\x22/search?hl\x3den\x26amp;q\x3drelated:blog.stackoverflow.com/+Stack+Overflow\x26amp;tbo\x3d1\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CDAQHzAD\x22\x3eSimilar\x3c/a\x3e\x3c/span\x3e\x3c/span\x3e\x3c/div\x3e\x3c!--n--\x3ex27,_ss);x3c/scriptx3e"
}/*""*/{
e: "j9iHTLXlLNmXOJLQ3cMO",
c: 1,
u: "http://www.google.com/search?hlx3denx26expIdsx3d17259,24472,24923,25260,25901,25907,26095,26446,26530x26sugexpx3dldymlsx26xhrx3dtx26qx3dStack%20Overflowx26cpx3d6x26pfx3dpx26sclientx3dpsyx26aqx3dfx26aqix3dg4g-o1x26aqlx3dx26oqx3dStack+x26gs_rfaix3dx26pbxx3d1x26fpx3df97fdf10596ae095x26tchx3d1x26echx3d1x26psix3dj9iHTO3xBo2CONvDzaEO12839712156911",
d: "x3cscriptx3eje.pa(_loc, x27rsox27, x27\x3c!--m--\x3e\x3cli class\x3dg style\x3d\x22margin-left:16px\x22\x3e\x3ch3 class\x3d\x22r hcw\x22\x3e\x3ca href\x3d\x22http://blog.stackoverflow.com/category/podcasts/\x22 class\x3dl onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x275\x27,\x27AFQjCNGnAJSxUa4GLcg-E7PNvIFmPC53gQ\x27,\x27\x27,\x270CDEQFjAE\x27)\x22\x3epodcasts - Blog – \x3cem\x3eStack Overflow\x3c/em\x3e\x3c/a\x3e\x3c/h3\x3e\x3cdiv class\x3d\x22s hc\x22\x3eJoel and Jeff sit down with our new community coordinator, Robert Cartaino, to record a “bonus” podcast discussing the future of \x3cem\x3eStack Overflow\x3c/em\x3e and Stack \x3cb\x3e...\x3c/b\x3e\x3cbr\x3e\x3cspan class\x3df\x3e\x3ccite\x3eblog.\x3cb\x3estackoverflow\x3c/b\x3e.com/category/podcasts/\x3c/cite\x3e - \x3cspan class\x3dgl\x3e\x3ca href\x3d\x22http://webcache.googleusercontent.com/search?q\x3dcache:JT0sWmmtiAEJ:blog.stackoverflow.com/category/podcasts/+Stack+Overflow\x26amp;cd\x3d5\x26amp;hl\x3den\x26amp;ct\x3dclnk\x22 onmousedown\x3d\x22return rwt(this,\x27\x27,\x27\x27,\x27\x27,\x275\x27,\x27AFQjCNErCiLBch55HA8i5BAdChcmQYH8nw\x27,\x27\x27,\x270CDMQIDAE\x27)\x22\x3eCached\x3c/a\x3e - \x3ca href\x3d\x22/search?hl\x3den\x26amp;q\x3drelated:blog.stackoverflow.com/category/podcasts/+Stack+Overflow\x26amp;tbo\x3d1\x26amp;sa\x3dX\x26amp;ei\x3dj9iHTLXlLNmXOJLQ3cMO\x26amp;sqi\x3d2\x26amp;ved\x3d0CDQQHzAE\x22\x3eSimilar\x3c/a\x3e\x3c/span\x3e\x3c/s
