Just as an object needs a subject, the person in attack is not taking an independent position but is acting as an assistant. After all, you need your opponent to complete the other half of a whole.
The gentleness/firmness is one inseparable force of one unceasing interplay of movement. If a person riding a bicycle wishes to go somewhere, he cannot pump on both [of] the pedals at the same time or not pump on them at all. In order to move forward he has to pump on one pedal and release the other. So the movement of going forward requires this “oneness” of pumping and releasing, and vice versa, each being the cause of the other.
— Sifu Bruce Lee