I learned how to do this movement at a seminar with Ted Wong in 1994. Its actual name is the floating punch or the short bridge punch. It can be done from an inch away or a couple of inches away, but the idea is to channel force through the body to the extremity, the hand, and torque it vertically at just the right moment.
The traditional Chinese artists used a form of internal force they referred to as "ching", to develop for a healthy body and general fitness. One manifestation of ching was used to channel into one protruding fist, creating an explosive force with no apparent torque behind it. This is considered by many who practice internal medicine to be an abuse of the internal system and hazardous to the health since the chi is localized so much. I have read about some saying that such an abuse can even make the practitioner's internal organs fail or explode.
The way Lee did it was a deviation from the traditional way. Actually the way he did it even when he learned WC from Yip Man used a small torque to add force. When he developed the small-phasic-bent-knee stance he was able to combine the floating punch he already practiced with the new footwork dynamics. If you look at the video footage of him doing the punch at an expo in 1968, you can clearly see him pushing off from the back foot really hard and taking a very small step with it if you look closely. This is the transfer of force from the ground to the hand.
The force from this move is not a push. It actually has penetrating force. Several people in the group I was in volunteered to be hit with (and two without) a pad in front of the chest and all of them complained about chest pains for nearly a week afterwards.
"After all, we all have only two arms and two legs."
"Death is certain, life is not."