The bottom does not get tucked in or under in training the form. Doing so weakens the lower body and the energy of the dantian. When the bottom tucks in the dantian is less able to drop.
When relaxed more energy is able to reach the dantian. The dantian stays level as if it were a bowl filled with water. The dantian fills with energy, when level the energy does not spill out.
Let the back follow its natural curve, do not go against the natural body.
Fold the kwa (hip joint) letting the dantian drop down or roll down and relax. Feel the fold of the kwa let the dantian drop or pull down, not up.
Even though the knees are open there is also the feeling of the knees closing in. The knees are held as if they are holding a ball or holding onto a horse. They remain open, and closed.
The body feels like it is sitting on a chair. The head goes up, the joints open. There is flexibility. The bottom is not tucked in and the hip joint is loose. It is ok for the bottom to go out a little, but not so that the body leans forward. Settle the movements, then drop the bottom. This will put less pressure on the knee and make for smoother flow of movements.
When the hip joint is loose the bottom and top parts of the body are able to remain flexible.