Ok so first spawn the model without the head, then right click the model in animation set editor. Tap add bodygroups, lets say you want the model without the hand. Tap left arm, then there should be some settings that say: normal,short, and long, long would be only the hand cut off while short would be up to his shoulder cut off. Now that you did that. The head is still blown up. Right click the model in animation set editor and make there no head, there won't even be any blood, then spawn a new model of the characters head. Either put the head model where the head is supposed to be and lock it or if you want it to be exactly where its supposed to be lock all the bones with the matching, example: lock the bip.head of the gibbed one to the bip.head of the head. Thats how you do it