How did the Trebuchet work?

The counterweight is dropped, swinging the beam toward the target. The sling, with the rock in it, is whipped skyward and forward. When the counterweight hits the bottom, the beam stops moving. The sling keeps moving skyward and forward. It swings the rock forward on a second axis multiplying the centrifugal force. When the rock approaches the apex of arc, the release pin is triggered. This lets go of one end of the sling, allowing the rock to fly free.