This trick is only possible in Co-op mode on Breaking Point. Get near a box of mortars and have a partner stand on top. There must be enemies around to kill him. When he goes down, walk over to him to see the message "Press and hold X to pick up and prime". Hold X and you will pick up a mortar and see the revival status bar begin to fill. When he is revived, you will put the mortar away as you would the needle.