As Nod says, it's a fairly simple trigger arrangement.
(Extract from Trap Making, Step by Step)
The trigger uses a long straight rod attached to the side of a strip of mesh or metal, which in turn is pivoted on the trap floor as a treadle. To set the trap, the door is swung up and the treadle tilted so that the end of the rod slides through the door and the roof of the trap together. Poked through both meshes like this the rod keeps the door open.
Bait is put at the back of the trap, well behind the treadle and not under it. Any animal trying to reach the bait will push or tread o