The trebuchet is a counterweight siege engine. Basically, a lever and a sling. If you understand these two concepts, the rest is just holding it together.

The Lever

The trebuchet is a Type-1 lever. In the Type-1 lever, the force is applied to one end, the load is on the other end and the fulcrum sits between the two. The playground see-saw is a Type-1 lever.

For the trebuchet, the force is very large and the load is very small. Like putting an elephant on one side of the see-saw and a small child on the other. You can imagine the result.

The other thing that happens is if you move the the fulcrum towards the force end (the elephant), the other end will move further. The elephant drops one foot and the small child end moves six feet.

The Sling

The sling is the oldest projectile weapon. There is a pouch to hold the projectile and two long strings. Both ends of the string are held in one hand and the sling is swung around and around. Then, at the proper moment, one eld of the sling is let go with the other end is still held by the hand. The pouch can no longer hold the projectile and the projectile continues on it's way. For the trebuchet, this release is done with a nearly straight hook so that the sling swings out and eventually slips off the hook, releasing the projectile.

The Trebuchet

So, putting it all together, the trebuchet starts with a long arm. The axle (fulcrum) is more towards one end than the other. The short end has a heavy counterweight and the other end has a sling. When it's "cocked and locked", the counterweight is high and ready to drop. The projectile sits in the pouch of the sling beneath the treb, ready to be pulled along and flung.

When the trigger is released, the counterweight begins to drop. This moves the other end of the arm very quickly and the projectile in the sling is dragged along underneath the machine. As the weight continues to drop and the arm continues to swing, eventually the projectile reaches the end of the base. Centrifical force holds the sling taunt as the sling swings out but the one end of the sling is still looped over the hook.

Forces try to straighten out the sling and eventually the loop on one end slips off the hook. The projectile is no longer held in the pouch and so continues on its way down range.