Davis packed things like phone books or a stack of magazines under his vest to help further spread the blunt force trauma.
True, but this was at a time when he was essentially a one-man road show spreading the gospel of soft body armor (then a relatively new concept). Getting hit is no fun. Deliberately hitting yourself several times a week while attempting to generate sales? I can understand him "cheating" a little bit. Otherwise, he'd have been on bed rest recovery after a just few demonstrations.
His graphically demonstrated point was that armor would save your life and that you could still fight back after being hit...a point proven repeatedly in later actual saves of law enforcement and military. He also dispelled the myth that bullets would physically knock you down.
I had an instructor who was not a fan of body armor (late '70s) tell our class about a Federal agent in Chicago who took a center chest hit from a .44 Magnum (on very early light armor) and whose sternum was supposedly split. Our "expert" served that anecdote up as proof that blunt trauma was completely disabling and we should not even consider wasting time with armor.
I asked him what he thought would have happened had the guy not been wearing it? The agent had survived the shooting, BTW.