How the knee-down revolution changed catching - without downgrading defense

Swanson initially thought that having a knee down might affect how umpires perceived pitches: Since umpires typically position themselves between the batter and the catcher’s inside shoulder, Swanson experimented with dropping the right knee for a left-handed batter and the left knee for a right-handed batter.

“I thought there was a chance umpires were using the catcher’s body, specifically their knees, to gauge the height of pitches,” he said. “I thought if we remove the knee from their visual field, maybe they’ll lose perception. … I was never able to validate the hypothesis.”

However, he found there were other benefits to catchers being closer to the ground: The best framers got most of their borderline strike calls in the lower portions of the zone. Moving the glove up to catch a ball at the bottom of the zone made it seem like the glove finished in the zone.

In 2017, among pitches taken by hitters in borderline areas - the “shadow zone,” as Baseball Savant defines it - 48.4% were called strikes with no runners on base, but the strike rate dropped to 45.1% with runners on, when catchers used higher, wider stances.

That data made a compelling enough argument to adopt a lower stance on all pitches, regardless of whether there were baserunners.