That is a great exersize that Bruce developed from the one I mentioned above which he learned from his Wing Chun master.He was a smart man and adapted exersizes from Fencing too which is even closer to boxing. That exersize he developed is more useful to his own style JKd and other fight styles that use all the body parts to fight with; like with head butts, finger strikes, knee breaks,elbows etc. Its limited in its use for Boxing in so much as the fight is only with the hands and is faster not in hand speed , but in the general fight movments only because there are less targets in boxing and less body parts used for weapons .
I think it may be of use to show up where your partner sees an opening that you are usually not aware of.
But it could be a lot harder to accentuate a slight bluff or parry or a slight body movment and expect an opponent to interupt his punching to the head or body target because of it.