I'm pretty sure your thumb is at the bottom of the mitt, but nice attempt at humor.
Not on a sinker to a right-handed catcher from a right-handed pitcher. As the ball sinks down and to the left of the catcher he moves the mitt to the side so the top of the mitt is facing slightly down and to the left. When he does that, the pinky points straight down to the ground. As they mentioned and I was pointing out is that the motion that puts the pinky straight down is pretty much unique to catching a sinkerball pitcher (or a left handed slider). Fastballs the mitt would be straight up. Curves or sliders the mitt would either be straight up, slightly to the right (putting the thumb down) or straight down if you had to block a ball in the dirt. But with the sinkerball you have to reach out to the left and down if it tails enough and that was what was beating up his finger.