The camera is actually doing a focus lock on it.. this is great as Will states to help you pick a focus spot and then move to a different spot to compose and frame your shot. Keep in mind that if your subject moves greatly, you will need to start over as the focus was locked specificially for that moment... it works well for trees, flowers and the occasional kid shot.. but once they fidget and move a lot.. you'll need to start over.

I've used this for moments when I knew something was coming, say a bike rider. I'd focus on a spot where I know the rider will be coming from.. and then just depress the rest of the way once they get there.. bingo, instant photo in focus with no delay. Of course, YMMV..

Great info Will!