[BerntR]

Hello no_mid_golfer,

I was a bit surprised by your reaction because I don't se any major disagreement with what you wrote. And I have no problem with the pump action works.

But the pumping up is very similar to what's happening when the shoulder rotates in the sketch I posted. Where the shoulder is pulling up. But I believe that pump-up as in the piston-driven experiment doesn't shifts the center of rotation in a way that provides a longitudinal force.

In any case, we seem to a gree om some very important basics here.