So many different opinions just by observing a single demo done decades ago. To me the one inch punch is more like a bicep hit. It's done to momentarily stun your opponent. The demo Bruce Lee was to show how much power the one inch punch can generate, although in practical use, it is merely a stunning move. The one inch punch does not have knock out power, but it does have the kind of power to distract an opponent while jerking him back slightly, enough space so you can launch your real attacks.
If you want a good description I suggest you look into Dan Inosanto's Definitive Collection of Jeet kune Do and Filipino martial arts. He goes over the 1 inch punch there, and who better than him interpret Bruce's teachings?
For those of you who think Bruce's version of the punch was to knock someone out. Plese consider the fact that Bruce Lee considered all ranges of fighting. understanding that his wing chun lost effectiveness once the opponenet was out of range, he incorporated boxing, because boxing was a sport, he incorporated muay thai because of the intesity they practiced with.
1 inch punch jerks your back a little, giving you range to throw a power punch. What good would the 1 inch punch do if it knocked your opponent on their behinds? The punch may have the power to knock them down...possibly. but it could not knock them out. Therefore Bruce used the punch to gain space between him and his target. In order to deliver a power shot.
Once again Dan Inosanto explains it in his Definitive Collection.
"Poor is the pupil who
does not surpass his
master" - Leonardo Da