I use crackhead and invisible collar a lot and it's all about the leg curls for the pump. I look at it like a winch or ratchet. The squeeze together moves the arm toward your centerline a little, the leg curl keeps it there. The next pump moves the arm a little more and the leg curl keeps it there again. The leg curls act like the locking mechanism of the winch to keep you from losing the progress you made in getting the elbow across. Just my perspective, but Coach Herzog uses the winch analogy for a lot of no-gi techniques and it seems applicable here as well.