[tbundy]

I believe the physics concept behind this is:

- An object will remain in the same state of motion (or rest) unless acted on by a force.

When you hit up on the bottle, you are applying a force primarily to the bottle. The force is applied to the sauce inside to a much lesser degree, so it stays closer to a state of rest. Therefore if the bottle goes up and the sauce stays at rest, it has the effect of moving the sauce down to the neck of the bottle.