In the context of taiji practice the body has three parts: the head, the shoulders, the hips. All of these must be level, like lake water. Each section of the body is level. The head is level; the shoulders are level, the hips are level. Feel how there is no tilting one way of the other.
It is important to connect the three parts; also it is important that the three parts are level by themselves and all together. We are instructed to drop the hips to connect the three parts and to assure that all parts are level. Dropping the hips is not the same as tilting the pelvis forward. Dropping the hips means letting the tail of the spine straighten and press gently in a downward fashion so that the arch in the back comes out slightly at the mingmen. When one drops the hips they loosen and this opens the bottom at the perineum and opens the front and the back. This will cause the back to make a natural arch so that the spine is straighter than its normal 's' curve. Doing this will allow chi to flow better to the ground.
At the same time slightly hollow the chest and arch the back like a bow. When the chest is hollow the chi can flow to the dantien. Only when the dantien is filled with energy does the energy disperse throughout the body. When there is no chi flow to the dantien the dantien cannot fill with energy and the chi cannot flow to the body.
When the body is relaxed an arch can be found across the shoulders with the chest in a little bit. Also an arch leads from the head to the buttocks as the bottom drops. This requires folding at the inguinal crease in the front of the body. When these arches are in effect, when the bottom drops the heels sink into the floor. The toes grab the floor. This anchors the body to the ground.