I was talking with my senior Wujifa school brother and I stumbled upon the realization that the customary American (U.S.) way of teaching "Bottom Heavy, Top Light" in Tai chi is at least not at all helpful to new students and at worst, completely misleading.
The typical American teaching goes something like this: Imagine a string tied to the crown of your head pulling you up. Imagine your weight sinking down like roots growing into the ground. What rubbish!
Unless you are a natural at creating immediate muscular, kinesthetic changes throughout your entire body just from using your imagination, then this method will not work. Sure, over time you can build a belief system where your imaginations "feel" like manifested corporeal reality but you are probably fooling yourself.
So what is bottom heavy, top light? Do I create these kinesthetic experiences from a concept or do I simply notice an experience and describe it this way? Does one feeling occur first or do they arise together? Is it easier to notice one or the other feelings first? Here's my current understanding.
First, you've got to learn how to "sit down" into your legs. See my earlier post Sink the Chi: How to Sit Down While Standing for a quick, seminar-ish trick way to feel this.
AFTER you can feel your weight BURNING in your legs even in a high stance (not a low horse stance) AND you can stay somewhat calm and relaxed with this burn, then you have a beginner's understanding of what I think is called "bottom is heavy".
Just as you need night to recognize day, hot to recognize cold, etc.... (all the Yin-Yang dualities), you need one to distinguish the other, so too in this practice you need to feel one feeling first before you can recognize the other.
Relaxing the upper body creates a feeling which may be described as "sinking" into the legs which results in what may be noticed as "heaviness" in the legs. Then, and only in contrast, since the upper body now does not feel the same way as the legs feel, does not feel "heavy", how then might we describe how the upper body feels in contrast to the legs? Not similarly heavy? Maybe... lighter? Ohhh, bottom is heavy, top is light.
In a philosophical sense, Yin Yang arose together. However, I could not create the simultaneous mutual arising of bottom is heavy, top is light through simple and mere imagination or visualization. I'm just not that good. I had to work and sweat for years on a mundane, corporeal, kinesthetic level to relax and build up the muscles in my legs to carry the weight I was holding in upper body tensions before I could recognize these distinguished feelings.
But wait, is there more? What if I relax more and "sink" more? Maybe my feeling of bottom heavy, top light will change. Maybe there is not just one feeling of bottom heavy, top light but rather just as the day changes from sunrise to sunset, as my view changes as I climb the mountain, so too does the feeling change; bottom heavier, top lighter. Not through imagining but through kinesthetic change.
For a very informative description of how this fits in the larger context of stance practice, listen to the recording Wujifa Basic Zhan Zhuang Practice and Models over on the Wujifa website.