I am working on a Search Application and using open source Search Engine Lucene. Lucene is returning results in few milliseconds. My question is related to the link given below.
If you have really fast search engine - it's quite easy to do, but requires lot of resources server-side.
Firstly, use tool like Firebug (in firefox) or Dragonfly (Opera) or similar to preview what happening when you change any char in search query.
Site is requesting urls like that: https://demos.netrics.com/ncc7/find_json?table_name=names1M&sensitivity=0.843&query=abc&callback=jsonp1330501624190&_=1330501901072 and server-side scripts returns JSON object, which is decoded and search results are updated client-side (using js).
Its not very complicated and difficult to do, but as I said - it'll require lot's of server resources when your app get popular.