Hi Rich,
I'm well thanks. Good question.
At first when practicing Vertical Power, it helps to just sink down the leg and then just rise up through the leg. We don't look at the duality, we focus on the sinking and rising, contraction and expansion, substantial and insubstantial.
Then when have done this for a while focus and the change points. The point when you have sunk completely into the foot and are about to release upwards and when you are at the top of the rise and are about to sink. If you stay relaxed you'll start to feel sinking in the rise and visa versa.
We are told in the internal martial arts to 'sink the muscles and raise the bones'.
For me personally, the easiest way to find it was at the moment that I'm sinking down into the foot. This sinking force is coming form the continual release through the body, we're 'sinking the muscles' but we don't stop when we reach the foot. We continue throughout all movement. So when we release up from the foot, the expansion upwards through the white tissue (ligaments, tendons and facia), happens while we are still sinking. It's as if there is a separation of the bones and the muscles. There is a rise in the sink and a sink within the rise.
With practice, you'll feel the rise coming up through the leg as you sink to. Sinking and rising support one another and are inseparable. This makes sense because all objects on the earth have a sinking and raising or Yin and Yang force in them that is in balance. Tai Chi and other arts like Xing Yi are just very good at helping us to really play with these forces.
If you find that you can't quite feel the two forces working at the same time, then go back a step and focus purely on just sinking and then just rising. Eventually you'll start to feel the opposite force.
This rising and falling is combined with the natural sinking of the Dan Tien through the Deng, into the feet and back up throughout the body.
The outcome of the balance between Yin and Yang is that we can maintain a root at all times. One foot or two feet it doesn't matter. It gives us a clean Peng force the is transmitted throughout the entire body, every aspect of the body is is motion.
To ensure that you don't sever the root, remain relaxed, don't force it and try to ensure that the rise and fall are occurring at the same time and are in balance. It can also help to do it wrong too! Play with feeling the duality and then go back to just rising and just sinking. You'll see after doing right, that it feels loose and separated and has no root.
This is just me explaining my understanding thus far. If you talk to my teacher about rising and falling. He has got the the point where they are in balance, they are harmonized. This can only come from practice.
Thanks Rich. Enjoy your training,
Rich.