The Increment Search and Incremental Search Backward commands invoke the incremental search mode. By default, F12 is the Incremental Search command, and Shift+F12 is Incremental Search Backwards.
Once in incremental search mode, Source Insight will start finding matches as you type characters, starting at the current cursor position. As you type more characters, the search will become more specific. The characters you type will appear at the bottom in the status bar.
You can exit the incremental search mode with any command key (such as an arrow key) or by pressing Esc. If you want to search again, just type F12 twice - it will load the old pattern and find again.