This video illustrate that gears are not always round, as long as there is good connection between two rotating objects, then transfer of power will happen. Having good connection is the key of generating in the body for taijiquan purpose.
The main connections are leg – torso and torso – arm. Torso – arm is a simpler connection and can be experienced in Six sealing foundation, where torso rotate away from the arm trajectory.
Torso – legs is more complex because there are two kua at the bottom and torso at the top. This can be experienced in twist towel foundation where both legs are used to power the torso.