This comment was posted by Tai Chi Xin a few days ago in this blog:
"I would like to add that a good Tai Chi movement should be developed through three stages:
1. From "hard" to "empty": relax the bones, upright postures, reducing the hardness of the body.
2. From "empty" to "having": after reducing the hardness of the body, one gets the softness, then relax the muscle and extend the bones, Qi is filled with the body, keep eight balances - Top and Down, Left and Right, Front and Back, and Inside and Outside.
3. From "having" to "empty" again: this is called "Hide"."