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Propulsion in a balloon car is very similar to rocket propulsion. When a balloon is full, the air inside of it is pushing against all sides of the interior of the balloon. Think of it like this: there is too much air in the balloon, and it wants to get out, so it pushes against the sides in all directions. When you make an opening in the balloon, that “push” can finally go somewhere, and the air exits the balloon at the opening. Remember, though, the “push” was originally in all directions, but the opening only lets out air in one direction, so there is now more of a “push” left on the side opposite of the opening. This means that the car will travel in the direction of the leftover “push,” and opposite to that of the opening.