Trebuchet Basics 
Trebuchet Cocked and Ready to Fire
A trebuchet consists of five basic parts: the frame, counterweight, beam, sling and guide chute. 

The frame supports the other components and provides a raised platform from which to drop the counterweight. 

The counterweight, pulled by gravity alone, rotates the beam. The beam pulls the sling. 

The guide chute guides the sling through the frame and supports the enclosed projectile until acceleration is sufficient to hold it in the sling. 

The sling accelerates and holds the projectile until release. 
 

How the Beam  Accelerates the Projectile
Trebuchet Sling and Projectile Leaving the Guide Chute
One end of the sling is fixed to the end of the beam, while the other is tied in a loop and slipped over a release pin extending from the end of the beam. As the beam rotates, it pulls the sling, with its enclosed projectile, down the guide chute. As the sling exits the chute, it accelerates in an arc away from the beam, but because the beam is still pulling the sling behind, the loop is held on the pin.
How the Sling Releases the Projectile
Trebuchet Sling Release Angle
The sling continues accelerating through its arc until it eventually swings ahead of the release pin. At this point, known as the release angle, the loop slips off the pin and the sling opens...
... releasing the projectile.

Trebuchet Projectile Release