Quote - > Quote - Other mystery are the joint and twist channel dials, normally hidden. For what do they exist ?

The actual dials (keys values) of joint and twist channels don't do anything. However the joint twist and smoothScale channels do a lot, they are the "Joint Parameters" and control how the mesh deforms (bends) in response to joint rotation.

That is the problem, the key value do nothing, but why do they exist?

Imagine if it would be possible to control the center of rotation !!!

Stupidity also evolves!