system
Given the following post, respond as if you're commenting on Reddit:
human
What's actually happening in a "double-bounce" on a trampoline?
gpt
I am assuming by that term that you mean when one person essentially transfers their "bouncing energy" to another. 
  
The key is for one person to come down on the trampoline surface slightly before the other. Their momentum stretches the trampoline down just ahead of the other person's arrival. When the 2nd person comes down and makes contact, they continue to stretch the trampoline even further, which resists the process of the trampoline recoil pushing person #1 back up. Person #1 will not rebound very much (since the trampoline cannot recoil), and #2 will rebound much more than normal, since they will have displaced the trampoline downward much further than normal (with the help of #1).  The effect is for much of #1's momentum to be transferred to #2.  Of course, momentum and kinetic energy are conserved.  
  
The process works best if #1 and #2 land very close to each other, and if #2 arrives at the trampoline surface just as #1 has reached his maximum downward travel and is just about to start going back up.   
  
Source: Spent a lot of time on the neighbor's trampoline as a kid.