When the legs bend, they compress a springy structure containing resilin – an extremely elastic protein that beats the best rubbers made by humans. Some type of latch, be it a physical structure or an opposing muscle, holds the leg in place until the right moment. When the latch is released, the energy stored in the spring is released, catapulting the leg outwards. The leaproach probably uses a similar mechanism, and Picker certainly found resilin in the joint between its femur and tibia.