The end aim of the Wing Chun kick is to stamp. It is in line with, and can be regarded as a direct result of, one of the foundational principles of Wing Chun, the facing intent. This would, in the immediate sense, mean that you “face” the opponent’s incoming kick and due to maintaining the “facing”, the kick you return is logically a “stamp” (at least wanting to stamp); any blocking contact at other parts of your leg (typically the lower leg, particularly the shin) is just transitional before the actual (finally landed) or potential (not landed) stamp.
When the opponent kicks you from a distance, the common response is to capture the trajectory of his shin so that you can block his shin to stop his kick, probably by using any part of the leg except the foot palm; or if you are skilful enough, by holding and grabbing his lower leg at his kick’s weak moments. However, relatively speaking, both of them are just passive defences, and the passivity arises, arguably, from the absence of the stamping intent.
Not capturing the opponent’s shin trajectory, and with the end aim to stamp, the intent will instead bring you to step forward (make it in range) to stamp onto his pelvis/thigh area before his kicking shin manages to travel the long range to hit on your body. The movement of the pelvis/thigh area is where you should monitor, rather than the shin.
Shooting your stamp from the floor to the opponent’s pelvis/thigh area is direct and fast, akin to the straight-line punch.
The opponent’s long-range kick looks fast if you focus on his shin; it’s actually not that fast at his pelvis/thigh area, of which the movement is relatively confined within an observable span even when his shin flies – the stamp can already destroy his long-range kicking momentum right there.
This still applies even if the opponent rotates his body to spin-kick, or jumps up to fly-kick. No matter how sudden he rotates, how high and forward he jumps, the span of movement of his pelvis/thigh area seldom escapes the detectable range. Just focus on his pelvis-thigh area, not the trajectory of his shin, you will then react faster.
For such long-range kicking, after you shoot a stamp (say with the right leg), return it to the floor with the flapping intent (not necessarily in straight leg) as practised in Chum Kiu, and then shoot another stamp with the other leg (the left), and so on as needed.
The flapping intent sustains the kicking power in the returning leg, leaving no weak moments exposed. This constitutes a continuous stamping, akin to continuous punching, which protects you when approaching the opponent from long range to close range. In reality, it won’t need too many kicks to reach close range though – 2 or 3 perhaps.
The stamping intent helps maintain your control zone. The control zone is drawn by the hip joint navigating the knees to any position along 360d clockwise and 360d anti-clockwise, to the largest extent (due to the flexibility in the hip) in all directions, but always making the knee CONVERGING.
Physically different from the control zone of the upper body, the control zone of the lower body must have one leg rest on the floor while the other kicks. Yet the intent is the same: the control zone changes its shape, concave or convex, as necessary to make contact with the opponent’s limbs, expelling them at its boundary. That is, when you right-kick, the left knee still converges and makes the left leg the left boundary of the control zone. When it’s the left’s turn to kick, this control zone changes its shape with the left boundary expanding towards the opponent while the right towards the floor. In both cases, they expel the opponent’s force once in contact.