jQuery Forum

In the example, search.php is the file name of the server code that manages data in, query performed, and data back out in JSON format to Autocomplete in the client.  Here's a tutorial about server code to support Autocomplete  - in several languages.

http://www.jensbits.com/2010/05/29/using-jquery-autocomplete-to-populate-another-autocomplete-asp-net-coldfusion-and-php-examples/

I don't understand your question about a trigger.  Autocomplete knows how and when to contact the server for data - when it is set up correctly, of course.