My Teacher, Master Cheuk Fung (pictured to the left), almost always started off his public classes with a 'pushing the sky' exercise done either in parallel step or with a weighted splitting component. Since there are no warm ups in Yi Chuan (you are either on or off) it eventually struck me a curious as to his choice of entrée.
In retrospect I can now see it. Vertical linkage (unifying oneself from head to toe) is a prerequisite to achieving the state of whole body unity. You could say that up & down are the mother and father of Integrated Strength, the two principal vectors among the six.
This makes perfect sense if you think about it. Gravity establishes our vertical axis, stretched between heaven and earth you could say rests our meat wagon. Without it we would be floating around like astronauts nearly helpless in a sleeve that's designed to work in a gravitational field, one that harvests the ceaseless pull of our planet for locomotion, a gravity sail if you will.