Differentiate substantial and insubstantial in order to string the body together without any breaks.
虛實宜分清楚，一處有一處虛實，處處總此一虛實，周身節節貫串，無令絲毫間斷耳。The Theory of Taijiquan by Zhang Sanfeng
The key is to remember that substantial & insubstantial coexist in the same space as two distinct qualities of the same thing. At first we will do some exercises that appear to separate substantial and insubstantial. This is a stepping stone. It’s not entirely accurate.
Shifting the weight
Stand in a neutral stance. Shift all your weight to the right leg. Relax and wiggle the left leg. Feel the weight, pressure and fullness of the right leg (substantial) and feel the loose light emptiness of the the left leg (insubstantial.)
Slowly shift your weight to the left leg and feel the change in substantial and insubstantial from one to the other.
String the Legs Together
To do this well your knees, hips, and ankles must be relaxed and connected. Imagine your joints are strung together like a marionette. As the upper body shifts from one side to the other, your lower body moves because it is connected. Keep the joints loose and relaxed. Feel how the looseness of the hips allows the weight to shift easily from one leg to the other.
As you work through the exercises below keep in mind that this body connection is a key part of what is happening.
Empty and Full
Continue shifting your weight back and forth slowly. Feel the sensation of weight pour from one leg to the other as one leg empties and the other fills.
As you walk your legs are always in a state of transition. Partially full & empty, shifting back and forth. Pay attention to this as you walk through your form.
Once you become familiar with this sensation of empty and full, begin looking for it in your arms and then in the rest of your body. It will be much more subtle. Take the time to feel it and differentiate.
Separate Upper and Lower
Relax and let your weight sink into the lower body. Everything above the waist should be light and fluid. While everything below the waist should feel very full and heavy.
Practice this enough that the upper body is very light and the lower is very heavy. Make sure you are still applying Tai Chi rules 1, 4 & 5.
Substantial & Insubstantial at the same time.
When done correctly the weight and power of the lower body will be transmitted through the light softness of the upper. At an intermediate level you can begin to differentiate between the soft emptiness of the upper body and the heavy power that resides within the emptiness.
With practice, let the heaviness drop down out of the legs so that the entire body is soft and light from head to toe and begin to feel the weight and power that resides within the softness.
For now, focus on the beginning and get really good at distinguishing between empty and full with the shifting of your weight. The better you get at feeling the subtleties of what’s happening inside your body the better you will be at differentiating substantial and insubstantial.