It indisputably works. The ball is flying between 85-100 MPH, cutting, curving, diving, running, dropping, etc. Umps can only get such a good read on where exactly it crosses the plate. A catcher who absorbs a pitch effortlessly and with minimal movement can instantly shift it a couple inches over the corner? Let the ball travel a little further to fall into the zone? Go and get it as to not allow it to drop out of the zone? You won't even notice as an umpire.
Also, as a guy who did catch in independent pro ball. I can tell you that umps will often literally tell you what they do and don't like with pitch presentation. "Don't turn the glove around, just catch it and stick it."... "Let those breaking balls come to you a bit more and they'll start turning into strikes."... etc.
Think about a "get me over" kind of a curve ball that drops into the zone. If you reach and extend your elbow upward to go and get it? You're going to catch that ball high and it will look out of the zone. If you let it come down to your face/chest. That's going to get called a strike.
Same with a low pitch. A ball crosses the middle of the plate, he sees that it's center cut... but the ump can't really tell EXACTLY how high it is off of the ground. If you get low and extend out/up with a stiff arm. Get slightly underneath it. You're going to catch it at a higher point. It's going to look like it is in the zone more than it would if you let it travel and you catch it nearly at the ground.
Umps are only human. They're as susceptible to not seeing the difference of an inch or two on a 95 MPH pitch as a batter.