There are two routes you could take... Probably simplest is to use two overlayed figures, occupying the same space. Set the arm to "not visible" in one model and everything but the arm to "not visible" in the other model. Or... use one model upto the frame where "separation" occurs. In this last "together" frame, export the arm an an obj and re-import it. Make the original arm "not visible" and then the severed limb can drop to the floor. The blood could be difficult, though!! H:)