The WC Tan Shou is quite unique. It doesn't exist in may other CMA systems such as long fist, praying mantis, Baji, Taiji, XingYi, ... The Tan Shou by itself drew my interest into WC many years ago.
To me, the concept of Tan Shou is to
1. extend my left arm between my opponent's right arm and his head.
2. extend my right arm between his left arm and his head.
If I can do
- 1, I can separate his right arm away from his body.
- 2, I can separate his left arm away from his body.
If I can do both 1 and 2, I can "separate" both of his arms away from his body and "occupy his center".