Re: punching chest level
If you are a tall get and want to really keep a guy off you with your jab, get low (by widening your stance) and jab his chest. He can't slip around it and he can't reach you. Watch the Bob Foster vs Dick Tiger fight.
If you are shorter you can do it to get a guy to throw his right hand at you, so you can counter it. Shade to your right (assuming two orthodox fighters), keep your weight back, and jab his body. He'll see the opening and throw the punch; he'll have to punch across himself because you are pulling to your right side. This opens up the counter.
Also, with the current infatuation with being square up and high guards, there is often a lane right down the middle for a stiff jab to the solar plexus. And the straight right counter to the same spot is a classic.