No idea, but it fools absolutely no one. He stands up so early that hitters know it's coming, and it is almost never a competitive pitch (just out of the zone). It's a ball from the moment it leaves the pitcher's hand, therefore it isn't even setting something else up.
Leon calls for those pitches, but he usually just lifts the target a little higher rather than standing up.
What I don't get it there is nothing sacred about using the glove as the target. A pitcher can throw to a knee or a shoulder or a whatever. All that's required is that the catcher knows that's where it's going.