My master told me that there are 18 "balls" in the body that one can rotate. The "balls" are actually joints, and the number of joints also matches the number of meridians. I don't know the location of these joints, so I looked up what meridians are, and found this link to be a good read:
I felt that the material described in the above link was relevant to Taiji, which was a practical way to improve Chi in our bodies. However, it was a by-product of practising Taiji, which main purpose was to fight.
Maps of Meridians:
What meridians are as found in more recent western studies: http://www.massagereferences.com/articles/Techniques/Fascia.pdf