If you watch the [entire video](https://www.youtube.com/watch?v=Ldgp3Ton7R4
) it makes a whole lot more sense. Before they put the ping pong balls in the garbage can they first put a bucket of warm water and a 2-liter soda bottle with liquid nitrogen in it and the cap sealed. Warm water evaporates the nitrogen, builds pressure, and boom.
ETA: link to video
I remember when i was young i had 1 ping pong ball and i thought they were expensive cause no one would get me another one. Anyways i loved playing with it and made sure not to loose it.
My trip to Thailand seems a whole lot less exciting now.