This is Bruce Lee's most common move and a favourite of martial artists everywhere. Unlike the straight punch which, in a combat situation, runs the risk of damaging knuckles or breaking the fingers if there is improper form or things go a little wrong at the impact point, a backfist always makes contact with the foremost part of the knuckle safeguarding the integrity of the hand and protecting the relatively fragile fingers. The trick is to throw a backfist with a whiplash motion. Remember the old school dressing room towel fight? Towels would be flung out and pulled back allowing the power of the motion to travel along their tip which is what gives off that painful sting when it hits you. The idea is the same with a backfist. Instead of a towel you are using your arm. Bruce Lee favoured keeping the wrist relaxed while the fist was closed giving an added whiplash to the punch and further increasing its power through the flick of the wrist. Notice that there is body movement here as well. Basically, like with any strong punch, the whole body is used to power it. Whip your arm back as soon as you're done punching.
What's involved: Triceps, hips, shoulders.
Powered by: Arm (tricep), hip flexors (both side and front), deltoids (the muscle on top of your shoulder).
Tip: Be relaxed. This is a whiplash-like movement. The greater the whiplash effect the greater the power, but it does actually require your body to be relaxed.