In the Wu Style Taijiquan of Wu Chuan Yau & Wu Chien Chuan, why do the practitioners sometimes "lean forward" in their movements?
From what I've learned, the idea is that when someone attacks the Wu practitioner remains rooted, weight in front foot, and subtly changes his position so that the attacker pushes themselves. It was a new idea to me, and I think, requires a lot of time to learn.
Also, it seens the strikes and pushes come from a rolling from rear foot to front.
And in throws the upper body is used as a pendulum in Wu Style
I will try give you my understanding from the Wu Style perspective (the style that I practice). As you probably know, Wu evolved from Yang (which in turn evolved from Chen). One of its main divergences from Yang is in the footwork, rooting and weight transfer. There are very specific reasons that these changes were made. Even though Wu also has a 50/50 weight distribution horse-riding stance, and a feet-parallel-shoulder-width-apart stance, I will mostly be focusing on the "sitting stance" (weight on back leg) and "archer stance" (weight on forward leg) combination. Most of the steps and weight transfers in the Wu form happen by using these principles. Each of the following individual changes have a cummulative effect in terms of the rooting that is possible.
- Weight distribution 100/0 - Unlike 70/30 in Yang, Wu uses 100/0 weight distribution. The "yin" weighted leg essentially carries all of the body weight vertically. This has the advantage increasing the pressure on the bottom of the foot and aiding in vertical compression (which is the basis for stability and energy storage). The yang leg carries almost no weight and is thus easier to move (because you do not have to shift the final 30% of your weight off of it in order to lift it.). This saves time and makes you more nimble. It also does not telegraph you intentions to the opponent. So you always operate with the yin leg as the "stake" (through which your compression happens), and the yang leg as the "post" (which simply adds structural strength and supports the other side of your body).
- Parallel feet - Both feet face forward and are shoulder width apart. If you shift to the back foot in this manner, you will feel a spiralling twisting through your whole leg that allows more energy storage. If you put your back foot out at a 45 degree angle, you will feel less of this twisting tension. If your back foot is at a 90 degree angle, you will store even less energy because your calf and lower leg are not really activated at all (most of the action happens in your quadraceps).
- Natural step size - Wu style uses a natural step size to go from one foot to the other. If you take a wider "lunge" stance, it will take you longer to reestablish compression over the new leg during a weight transfer (because it will take longer for your main body mass to get above that leg). It also requires more leg strength to execute. The resulting area between the feet is slightly smaller than the wider "Wu box", giving you a smaller overall base area. Having feet shoulder-width apart also lets you balance left and right as you shift weight or step. By contrast it is very difficult to balance laterally in a very deep and skinny stance.
- Staying within your base - Since the "Wu box" is smaller in the forward and back direction than comparable Yang/Chen stances, a couple of innovations have been developed to be effective and still stay within your base. In the forward archer stance, the torso leans forward at the same angle as the back "post" leg (there should be a straight line from the back heel to the back of the head). This allows longer forward reach. The top of the body is counterbalanced by the back leg, and all of the weight is balanced on the forward leg (so it stays at the front of the base but still within it). Similarly, if your butt ends up behind your rear heel-line in the back sitting stance you will fall out of the smallish "Wu box". This is prevented by raising the toes on your straight forward ("post" or yang) foot off of the ground while keeping your heel firmly planted. This causes a stretch from your toes, around your heel, though your calf and hamstring, and through your butt into your lower back. This taut "bow" supports it's side of the body and allows your hip joint to go back even further (enhancing your neutralization range-of-motion).