When teaching catchers, first I always preach getting the strike or catching the ball first. Its a no brainer. But an exceptional receiver can begin to load his feet while keeping the mitt in place. Watch I-rod or the Molina's. Check many catchers on tv with a runner on and a foul ball is hit. During the slowmo replay you can see then react to the runner breaking before they have the ball. This is very hard to teach. You have to recognize the kids who have the ability to do it.