This is the Medieval Atomic Bomb trick thing here. Its pretty cool, it shows how a trebuchet can launch the projectile from any point TO any point as far as you want and cause a bunch and chaos and stuff. Its not that complicated but, the timing can be.
This utility features a trebushet attacking a village half the map away.
UTILITY: This utility shows a cool trick that can be used in a scenario. It can be used in a siege, or it could represent a magic spell. It also shows a neat explosion. 5
ORIGINALITY: This is not a brand new discovery. It is a trick, and is not very hard to recreate, as you mention yourself. I have not seen that trick done with a trebushet though, and that is a plus for you. 4
SUGGESTIONS: You should have included several ways to use this trick. Maybe several trebushets firing at a fortress; maybe a little game featuring taking a fortress. Be creative!
DESCRIPTION: You said that it was easy to make, and the only thing that was hard was the timing, and yet you never describe it. 4
SUGGESTIONS: Your utility is supposed to teach the player something. That being said, describe the relationship between the time when the trebushet should fire, and the distance between it and its target.