Search as you type is introduced in the search tool as a feature. Searching as you type means that you see the search results before hitting enter and you can decide if more letters need to be entered in order to refine further those results. It's the equivalent of Google's search box drop down.

Of course the search tool does not perform such search before hitting enter if the file in question is too large, because it would imply a processing time deemed too long to make the user interface useable.