To activate focus lock with most digital cameras, the photographer centers the subject in the frame and then depresses the shutter button halfway. While keeping the button partially depressed, you can move the subjects out of the center of the frame, and the camera will keep the subjects in focus. Or you can wait to snap the photo until the subjects perform the action you're desiring. When it's time to take the photo, just depress the shutter button the remainder of the way. With the camera already focused, it can respond more quickly.