One of the wing chun kuen kuit (or wing chun aphorisms) says that the fist comes from the heart (Kuen Yao Sum Faat). This statement has several readings, some more literal and others more metaphorical, but just as valid.
The fist comes from the heart because both during the practice of the form and the actual application, the fist is launched from the center of the chest, where we intuitively place the heart.
In traditional Chinese culture the heart is the seat of mind and conscious intention. So another interpretation of this aphorism is that the act of throwing the fist is more a consequence of an intention directed by the mind than an act of brute force. This interpretation lends itself to a reading that comes close to quasi-esoteric points of view that I do not share. I think that to speak of qi or hei in Cantonese, as of an invisible and mystical force that guides our movements, only distances us from martial efficacy. The Jing or relaxed force is a very real concept and it allows our strokes to be of great power, but there is nothing mysterious about it. When we say that the fist comes from the heart, from the intention, we must understand that the movement must be performed as relaxed as possible. When we say that we do not use force, we must understand that what we do not use is the strength of the muscles we do not need. Throwing a fist with a strained arm only slows us down and therefore rest power: the resistance of the biceps tension must be overcome by the triceps to stretch the arm. The relaxed force is achieved using only the muscles needed to make the desired movement.
Finally, for me the fist is born from the heart because the path of the Wing Chun is after all an inner path. Beware, I do not want to deny the first and most obvious utility of Wing Chun: fighting, martial art, self-defense. But without denying this reality I want to point to a deeper one, the one that slowly but inexorably changes you. The fist comes out of the heart, and with each fist the heart is transformed, our character is molded: that with a weak mood is strengthened a little, and the one dominated by anger is appeased. The central line that we look for in the outside is also created in the interior, a center that is balance between opposites without getting to annul them, but that is able to transit between the soft and the hard when it is necessary. Each year of practice leaves a mark on our interior, like the rings of trees that denote their age, so our interior matures with practice.
The fist is born from the heart, and from the fist is born a little more noble heart.