They don't go all the way down and touch their chest for the same reason they don't fully lockout. Continuous tension, TUT, Time Under Tension. It's actually harder that way as your muscles are always under tension. When you touch your chest there is a slight pause/rest as well as the bit of the bounce you get coming off your chest. Jay actually pauses a bit on the bottom position, still supporting the whole weight, for a couple of reps which takes all momentum out of the movement. Just pure muscular contraction. It's not reps per se that causes inroads into the muscles but the actual time under tension. Those reps are about as good as you can get for bbing purposes.
You can say that, but Jay never had the chest of Arnold or Lou Ferrigno, that chest that's so massive that it just simply sags because there's so much muscle mass in there, also he never had the chest of Ronnie, who also always went all the way down, Jay did have amazing back, arms, legs, etc, his only true lagging body part was his chest, he improved his chest to a degree, but he never had that monster chest look, that droopy look, where the chest literally sags, like a balloon filled with red meat
