A squib, which is a small explosive charge triggered by a 9V battery, was taped to the plate and a blood pack placed over the charge and secured in place with gaffer tape . Placing the blood pack over the charge reduces the flash given off by the squib detonating. The rigged plate is taped or strapped to the actor and concealed under clothing. White clothes show the effect off best and should be pre-scored with a knife to weaken the fabric. Ideally the squib should blow through the shirt and it helps here if the shirt is tucked in.
Cable is run from the squib down the actor's leg to a control box or nail board.
Before firing everyone stood well clear of the charges, the actor avoided looking down at his chest, and the camera was a safe distance away. Only at the last moment was the circuit completed and the charges made live. A simple countdown helped the actor be aware of when to expect the hit. Take a look at the film clip again. That is the first and last time I am ever going to use an explosive charge to perform a bullet hit like that.