In position 1 above, is the pulled back the arms is now stored as ‘potential energy in the tendon springs’. The arms are pulling very hard on the string which is stretched very tight. A locking mechanism (plate 27A) holds the string in place, just like in the flea.
In 2 above, the locking trigger (plate 27A) has been released and the arms are being swung around by the unwinding tendon springs. As they move forward they pull on the string. The string pushes on the stone ball, accelerating it greatly.
In 3 position, the string is straight, the arms have stopped moving, and all the energy that was stored in the tendon springs has been transferred into the projectile (big word for rock or bullet or arrow or whatever). The rock leaves the catapult at a very high velocity.
