From Kaplamino:

The air canon with the balloon was a big challenge to make. The rubber band squeezes the balloon around the straw to release the air in a specific direction. Then the hardest part was to find how to stop the air and deliver it when I want. It was too difficult to put something like tape at the end of the straw because it was either too clingy or not enough.

The solution was to twist the balloon around the straw and block it in this position with a rope. In this position the air stays in the balloon until it untwists. I explain it because I think what exactly happened it’s not easy to get when you see the video.

[Kaplamino | Via BoingBoing]