It is not easy to "classify" this kind of devices, you are thinking of the (more common) "counterweight trebuchet", but there were other kinds of trebuchets, pr

It is not easy to "classify" this kind of devices, you are thinking of the (more common) "counterweight trebuchet", but there were other kinds of trebuchets, propelled by (direct or indirect) traction or by torsion (similarly to the ballista). https://en.wikipedia.org/wiki/Trebuchet https://en.wikipedia.org/wiki/Trebuchet https://exarc.net/issue-2018-3/at/traction-trebuchet https://exarc.net/issue-2018-3/at/traction-trebuchet This seems like a form of traction trebuchet with traction provided by the elastic.