When you plant your ass on different kind of things with purpose as a trick.
The origin came from skateboardings all accidents when you fell on the ass, and the word assplant came upp naturally. Then it became a thing to jump around and "plant the ass" on different kinds of thing.
For example plant you ass on the roof of a car by jumping up and sit on it.
Or if you accidentally fell, you can say you made a assplant.