What does it mean to cut through the ball with your middle finger?

It’s sort of hard to explain but you sort of follow through with your middle finger and let it roll off your middle finger last. If you’re throwing a cutter, you end up basically pointing with your index finger.

I take it to mean that you maintain a set hand angle and impart force through the ball instead of around the ball. However, if you’re holding the ball off-center, you may till put some spin on the ball and get some movement. But you don’t change the hand angle as your arm accelerates forward.