LaoDan wrote:I am curious if practitioners have practiced this way – PH while standing on bricks – and if so, then what they were told the purpose was. To me it is a way to check that one is maintaining verticality into the feet, but I was never taught this method, so I could be wrong.
The photo shows two people (Yang Jwingming and other) fairly heavily leaning forward/bent at the waist. Your terminology of "maintaining verticality" will quickly become open to misinterpretation since it isn't being physically vertical that you are describing.
If a rigid object - say a cube - stands entirely vertically, its weight is entirely vertical against the ground - or whatever it is standing on. There is no horizontal component. If the rigid object leans, but still has the entirety of its base on the ground - such as the leaning tower of Pisa - the weight of the object is eccentric to its base of support. The entire weight of the object is still vertical, but it introduces a moment about the base. The moment, unless resisted, will cause the object to fall over (tip).
If a horizontal force is applied to the vertical rigid object, such as the cube, the weight of the object is still acting entirely vertically. The horizontal force, if applied to the object along the base of the object, where it is supported, the object will slide horizontally unless resisted by an equal and opposite frictional force. There is no vertical component introduced by the horizontal force: there is only the weight of the object acting vertically.
If a horizontal force is applied to the vertical rigid object, such as the cube, but is applied to the object above its base of support, the force is applied eccentrically, which introduces a moment that attempts to tip the object. In that situation, there is the vertically acting weight of the object, the eccentric horizontally applied force, the horizontal opposing frictional force and the tipping moment, with (or without) a countering equal and opposite moment from the support to the object. The horizontally applied force does not change the vertical forces acting on the object.
Where it begins to become interesting is that humans are not rigid objects and don't, generally, behave like one - though many beginner Taijiquan practitioners, and many of those being demonstrated upon, do. The interesting part is how can humans use the body to redirect horizontal forces into at least some component that is vertical. I believe that is what your discussion about "verticality" is about.
In the photo, above, the long axis of the bricks is aligned with the length of their stances. To tip the bricks end-for-end would require a considerable unbalancing of the person standing on them - if the bricks can be tipped end-for-end at all. Tipping them sideways, due to the small width of their base of support should be relatively easy. Tipping them sideways requires a moment about the width of the brick (also the width of their stances), rather than about the length of the brick (the length of their stances). Even with 100% of their weight remaining vertically directed, a horizontal force in the correct direction - producing the correct moment - will topple the bricks sideways. (People often discuss directing a force into one of the opponent's "gates".) That isn't about maintaining one's weight vertically.
(If one turned the bricks 90 degrees so that the width of the brick was in the same direction of the length of their stances, the interaction would change: one would win by pushing or pulling along the opponent's stance rather than across the opponent's stance. If one really wanted to test their skills, having each foot standing on one these might be a better indicator:
http://static1.1.sqspcdn.com/static/f/1 ... iVXt1qw%3D.)
That raises the question of whether or not what is needed is to simply ensure that the opponent cannot push horizontally in the direction needed to create the necessary moment. That is, if one redirects the applied horizontal force so that it does not act in the necessary direction to create the tipping moment, can one prevent being tipped? Alternatively, can all or a large portion of the applied horizontal force be redirected to act in a vertical manner, as another alternative to preventing forces in the direction of the vulnerable tipping moment? (One simple approach to that is to lower one's stance so that the opponent ends up pushing at least somewhat downward, adding a vertical component. Having a longer stance that one's opponent can produce a similar result, though it also changes leverage.)