An Ajax application eliminates the start-stop-start-stop nature of interaction on the Web by introducing an intermediary — an Ajax engine — between the user and the server. It seems like adding a layer to the application would make it less responsive, but the opposite is true.
I actually starting using this about four or five years ago (using Microsoft’s COM component in IE). I was really bad at it, though. Done wrong, this really kills usability — I was essentially letting a user browse an entire Web site without loading a new page, which is fine…until they try to hit the back button.