The force demonstrated when air rushing out of the neck of a balloon causes it to move is Newton's Third Law of Motion, also known as the law of action and reaction.

Here's how it works:

1. Action: When the air is forced out of the balloon's neck, it exerts a force in one direction (outward).

2. Reaction: In response to this action, the balloon experiences an equal and opposite force pushing it in the opposite direction. This is the force that propels the balloon forward.

Essentially, the balloon pushes the air out, and the air pushes back on the balloon, causing it to move.