The hinge point in behind the olive stack. You can see that there’s a bundle of sticks tied to the support there holding the log in place. The other end with the rock and pulley had a guy twisting a log in order to wind and shorten the rope attached the the rock. This causes the rock to pull down on that end, creating pressure on the olive stack as the other end of the log is anchored.

I’m guessing the tree is mostly a guide but the Y of the tree also provides a stop.