The principle of operation of a catapult is symmetric to a base holding a shaft capable of rotating together with a lever arm on this lever there is a support to hold the arm in order to disable rotation until it need and one end is the projectile launcher or holder.
At first, catapults they had a mechanism that allowed the lever arm stops abruptly upright, and that made the catapult project itself forward thus giving the name of this Onager catapult (donkey) because this movement resembled a kick. The problem with this is that big part of the energy that would be dissipated launch was in motion and cause vibrations in the structure which made it more susceptible to damage.
The trebuchet appeared in order to solve these problems. To get more energy, first made some considerations, for example, they could add a mechanism that optimizes the launch, it was deployed to the tip launcher cords that held a leather bag in which the projectile involved.
Another problem was solved recoil, in which locker was removed and allowed free passage of the lever leaving the lever movement being damped with external forces such as friction of the air with the passage of time. The shaft passed to a string strained solid shaft and at the other end a counterweight was added.