There's an arm that is wound back 180 degrees under spring tension. Operator pulls a handle and the arm swings 180 degrees forward and comes to a sudden hault. There is a pin at the end of the arm that holds the pigeons in place until they take flight. I can't find any descriptions out there either, but if you've ever seen one of these work, I don't know why it would be so hard to describe it since you're already in a class that probably gave you the necessary physics knowledge to do this.