You are photographing a football match. From a melee a shot is made at goal. You let the camera focus on the goalie and press the button. No matter how many players run across your field of view, you retain focus on the goalkeeper. Action over, release the button and resume normal AF. The main Olympus lenses have a button that falls right to the thumb for just this purpose. With Panasonic, try using half press shutter release with BBF. It's like a hair trigger on a rifle and makes the camera super responsive.