The early history of the crossbow

[Flavivs Aetivs]

I have the same issue, it varies from crossbow to crossbow but it seems to be similar to the simplest of Medieval crossbows. I'd imagine the Romans would put a spring into it so the trigger would snap back, I know they did it with doorknobs.

The Haute-Loire Arcuballista is the only known Depiction we have of one, and Maurice and Vegetius both give brief mentions. Arrian and Ammianus also refer to Ballistarii on Horseback. Comitatus has proven that works with the Arcuballista.