As a kid, I was always at my neighbor's house because they always had the newest and coolest trampoline. Turns out that this cool contraption requires many physics concepts in order to work. The energies required for a spring are kinetic energy, gravitational potential energy, and spring potential energy. When you bend your knees in order to take your first jump, you are using your gravitational force downward in order to compress the spring in the trampoline (work from your knees is transferred into spring potential energy). Then the spring releases, and the potential energy transfers into work done on your body, hence shooting you up into the air!