This is intuitive: A stance in martial art, apart from carrying the body weight, is to supply power to the body by drawing support from the floor, mostly through holding tightly with the toes and a wide separation of the legs. You may describe it as “rooting”. It is then natural that in order to initiate body momentum forward you need to “un-root” and “root” the feet alternately, which inevitably involves shift of your weight between the two legs. And this has evolved into a knowledge of stance techniques. Similarly, to kick with one leg, you need to “un-root” it first. That means at least one leg is always relatively “stagnant” at any moment and is paralysed (totally impossible) from kicking.
This is counterintuitive: In Chu-style Wing Chun, your body mass is the ONLY source providing power via its own momentum; there is no need to draw support from the floor. This is made possible by articulating your whole body onto your centre (loosely regarded as the centre of mass) inside, not via the muscular layer. That centre initiates body movement and thus momentum (not initiated by the legs from the floor). As such, the feet need not hold tightly the floor. Rather, the foot palms just rest (no toes grabbing) “evenly” on the floor in order to carry your body weight only. Because of no rooting, the two legs are equally free to move (in stepping or kicking) at any moment, without the need to shift weight between them. This is possible since the whole body has been articulated onto one centre as one entirety, the linkage from which already suffices to move one leg, just leaving the other leg resting on the floor to carry the body weight. In such state, you won’t feel “weight” pressing down on the legs, and you won’t need a stance to keep balance nor provide power. You feel your body light, as if nothing physically there. That is, “no stance”.
This is almost intuitive (taigong, sing and drop): Most people will agree that “taigong” and “sing” are the emphasis when training Chu-style Wing Chun. Briefly stated, you start from “taigong”, use it to rise (“sing”) the spine up to the top of the head as muscles at the back drop; at the same time the “taigong”, with the thigh muscles going up into the hip joints, rotates the pelvis and further drops muscles down to the knees which are focusing forward. In this way, your body will then be “vertically” connected and act as one object. This is the typical understanding which, if not comprehended in subtlety, often results in misuses. While the body is now one object in the vertical dimension, it does not automatically guarantee that an equivalent momentum can be effected on the horizontal. In most cases, such transfer of momentum from the vertical to the horizontal is done by leaning backward in order to press the weight towards the knees so that the originally vertical momentum is now obliquely applying to the knees to force them and hence the body forward, although the entire body is still being kept as one object by the erected spine.
We can view such transfer from vertical to horizontal as by using the horizontal component of the oblique downward force, which is itself changed from the original vertical momentum. To increase forward force, say in stepping, a stance of this type must first increase the oblique downward force, resulting in obvious kneeing movement aiming towards the floor, again obliquely. Such “forward” force is not hard to block or deflect because it is monotonous, mostly unidirectional, and thus its direction can be easily caught. This method is not a smart way of transferring power from vertical to horizontal, as it is not approaching the use of your entire mass on the horizontal.
This is further counter-intuitive (stacking): Let’s make things simpler to start afresh. First, don’t allow leaning backward, which usually goes in tandem with the tension in front of the hip joints holding tight the position. Second, don’t draw support from the floor, i.e. trust that you don’t need to feel “firm” at the foot contact with the floor in order to stand stably there; feeling loose, detached from the floor is even better for a stable stance. Third, it’s not a must to start from “taigong”, if this thinking appears not helping and has resulted in the manner described above. Instead, think of the stance as just stacking up different parts of your body vertically, one on the top of the other without anything to tie any two parts together. It is like stacking up separate cubes from the floor high up to the ceiling. The stack of cubes doesn’t fall apart because every cube “rests” satisfactorily on the cube below; there is no need to tie any two cubes in order to keep them together. This creates a vertical “virtual” alignment. It is virtual because there is no physical tying-together.
Applying the stacking to the body, you regard every joint along the vertical line breaks and is the junction between the parts above and below. Then, you have these parts stacking one on the other: lower leg on foot, thigh on lower leg, pelvis on thigh, torso on pelvis, neck on torso, head on neck. Try to achieve this mainly by soften-melting muscles into joints (leave aside active joint rotating for the time being) so that any two parts are just loosely linked (feeling-wise). Stacking in this way will build up a vertical “virtual” alignment, which you have to keep. Just keep, keep it vertical, not changing to oblique. There should be no leaning backward nor pulling forward in such vertical stacking, because there is no need to strike a balance back and forth on the horizontal. Just keep, keep it steady, not a must to increase its intensity. Being stable is more important than magnitude. Forget about the condition of “taigong”, the position of the tailbone, where the “sing” has risen to……., if they don’t help. Pay particular attention to the hip joints (muscles in its front soften-melt directly into the joints, not a must to consider the thigh muscles) and the knee joints (not locked up to stand firmly, but instead flexible or loose for stacking).
By stacking, the body is being kept as one entity by the vertical “virtual” alignment without any push and pull on the horizontal. When this one entity moves, its different parts are acting accordingly but without violating this principle. You can then imagine it as this “virtual” alignment (up to the head, down to the knees) being frictionlessly “floating” on the level of the two knees to give out momentum in all directions, be it forward, backward, or sideways. In this way, the original vertical momentum is being transferred to effect on the horizontal (obviously joint rotating is a key player, but we don’t go into details here), fully tapping into the whole body mass. Such transfer is readily available even in “still” stance: the “virtual” alignment is free to move a full circle (clockwise and anti-clockwise, though extremely small) on the level of the knees (floating), not noticeable at all but potentially ready in the mind. This is how the stable Wing Chun stance could be configured.