Often on the net and in person people often mention the “dan-tian” talking about moving from or driving movement from it. 
This is done assuming everyone agrees on, the what, how, and why something called a “dan-tian” is needed let alone something that should be developed and used.
An interesting site with information not widely known nor presented.
“ANATOMICAL LOCATION OF THE LOWER DANTIAN
The Lower Dantian is centered below the umbilicus, and inside the lower abdomen, forming a downward pointing triangle. Its boundaries are defined by the three lower chakra gates, described as follows:
Nine Chambers of Lower Datian for the female body
Huiyin (Meeting of the Yin): The lowest point of the Lower Dantian extends to the Huiyin CV-l point at the perineum. This point is located midway between the genitals (anterior Yin or Qianyin) and anus (posterior Yin or Houyin), traditionally known as the “Huiyin.”
The name “Huiyin” refers to the area on the body responsible for gathering and absorbing the Earth energy. This area is responsible for gathering the Yin energy into the body and Lower Dantian area via the three Yin leg channels (Liver, Spleen, and Kidney), and is the intersecting point for the Governing Vessel, Conception Vessel and Thrusting Vessels. This area is sometimes known as the lower gate of the Taiji Pole, or the bottom gate of the Lower Chakra”
Another way of looking at yin energy is “emptiness”
It mentions “taiji pole” lower point of the taiji pole
There are 3 points that must be aligned Bai Hui,Hui Yin, and yung chen.
They are called the inner alignment.
These alignments along with the three outer alignments shoulders, hips, ankle, are used to from a spherical body.
