The source is just my own eyes and the logic of it. In the shot it appears to me that after Mark Hamill lands, he then stands up from a crouched position. The scene cuts right when he reaches full stance. I have worked on films containing stunts and can't imagine that they would have him land on a trampoline for that scene. He most likely would have landed on some sort of padding that would not propel him back upward if it was not necessary. If you watch the scene without the landing sound effect and then see Hamill stand up, you can tell that the platform he jumps off of is about the same height as he is. I think the sound effect was meant to make the platform seem higher than it actually was.

Oh, and regarding the potato/ shoe thing... Although I can't quote the exact source (maybe it's the Ken Ralston or Dennis Muren commentary on the films), but I think the potato thing was in the Empire asteroid sequence only and they used someone's sneaker and maybe wads of gum for the VERY DISTANT battling ships seen through the Emperor's window in Jedi.