Even if the scene is a bit artistic, you really have to appreciate Bruce Lee's fighting style.
Watch his shoulders when he punches, especially from minute 2:38. They give no foreshadowing about the coming punch. His fists and forearms move first, then his shoulder, if at all. That makes it so fast and unpredictable.
You have to train that, which is not easy. Because most fighters move their shoulders first. You must keep the whole arm loose until the moment of impact to make it as fast as possible. And only in the last nanosecond the fist and the whole arm is tightened.