The diagram has omissions. Either: 1. There is a floor that the weights (and the man) are standing on. 2. Weight C is dropping whether he pulls the rope or no

The diagram has omissions. Either: 1. There is a floor that the weights (and the man) are standing on. 2. Weight C is dropping whether he pulls the rope or not. I think that the diagram is meant to show all the weights resting on a floor, and all the confusion is due to misunderstanding of that. In the other case, the diagram neglects to mention something to the effect of "supports have just been removed".