Stephen things that happen at impact are very small, with big impact on outcome.

I would say the trampoline is so tight you would never see the movement(with the naked eye) if it were possible to see under the ball.

But I would guess that the ball is relevant to how much the face smashes. A harder to compress ball would cause more compression of the face.

I'll take wild guess and say it deforms 5 micro-nano-millimeters, but if you change that to 4 or 6 it would be a big change.

They might be able to measure the deformation with some ultra sensitive pressure sensors inside the head.