Follow through

[Sparky Guy]

But isn't that what Finch does in the clip I posted?

Her elbow clearly stops for a frame or two, which helps transfer momentum.

I'm not sure this is a good idea for a younger pitcher, but I'm thinking I'd teach it to a pitcher who has stopped growing.

You don't see the whole finish of the pitch in that clip. As she speeds the hand and wrist through the release point to snap the wrist and turn the ball the elbow appears to stop but it doesn't. If you watch the last couple of frames the finger tips are pointed at the front hip with the palm up after the ball is gone and the arm is continuing forward as the clip ends letting the momentum dissipate naturally.

Notice that her arm is not straight. It has a slight bend to it so it can be used with a whipping type motion to accelerate and manipulate the ball.