Larry Holmes once told me throw the jab like a whip, don't worry about the power just look for speed/snap. He also said he sometimes held his left hand low, because when he threw the jab in that position.

Because boxers watch the shoulders in boxing aswell as other things, but throwing the jab in that position they actually couldn't see it coming even if they are watching the shoulders. Very risky and not something i would advise doing but i thought i would throw that in there.