Form without substance

I've been doing the Sun style form since my first lesson with my current teacher. After learning the whole form, it's been doing it over and over again. The one consistent message which keeps coming through is this - my form lacks substance.

I've been trying to figure out what this "substance" is. I think it's the peng quality that should be present at all times, yet my teacher's peng is so hidden, I only realise it when i'm already off balance in push hands. It could be the whole body movement thing. Whatever it is, I was brought to a realisation of what "no substance" is instead of what "substance" is.

One of my push hands buddy gave me an empty push - he was pushing with his hands rather than any movement of the body. And it was done with such speed that it gave the impression of overpowering. It was then I understood what does "no substance" mean. Like a child waving a branch at a cat, the dead branch does not have any substance. If the child plans to whip the cat with the branch, the branch will then contain a kind of tension waiting to be released upon contact - and that's substance.

From that child and branch analogy, I guess what I should be looking for is to have that kind of tension stored in the body at all times. Wouldn't that be tiring? I'm not sure. But I already know what the second step is - to hide the tension, which I suppose, I need to relax the whole body, which in turn might be the first step towards having that kind of hidden tension my teacher has. It sounds like relaxing is the key so that you only release that kind of tension when you need it.

Relax to have tension. The statement is an irony in itself. Am just chasing my own tail?