Ideally, imagine a ball poised high above an trampoline. When the ball is released and falls, it bounces and returns to it's original height due to the conservation of energy. Now, imagine the ball is a spacecraft and the trampoline is a planet. Instead of bouncing on the trampoline, it takes the path around the planet such that gravity will return it on a outgoing trajectory parallel to the incoming trajectory. I hear of trajectories around planets, such as Jupiter or Earth, that provide a speed boost due to a slingshot effect. How can this possibly add any additional energy/speed to the craft and not violate the conservation of energy with regards to the trampoline scenario? I'd imagine the outgoing velocity gained from the slingshot would only be enough to return the craft to it's original distance from the planet and no further.