If you don’t mind me stepping in, I can illustrate with simple pics:
Very simply elbow, middle loop weighted half and half. We suffer volume loss.
Add one fan bone, that splits the rotation, and weight fully to it instead of half-and-half:
No volume loss. Shown over original as proof.
Could use “volume preservation” but the fan bone lets us have tunable volume preservation, without discontinuity when mixing from volume preservation to no volume preservation. We can do whatever we want with these joint bones, rotating them differently, weighting only the inside or only the outside of the elbow to them, etc.
On a higher poly mesh, we might use more bones:
Spitting rotation between more bones, we see the inspiration for the name: it opens up like a fan.
This is essentially what we’re doing with bendy bone or spline IK fixes for joint problems. Not 100% the same, but we’re doing the same kind of thing, for the same kind of reasons.
We’re doing the same thing, spreading out that deformation along the joint into many small deformations, and using more bones so that vertices can be weighted more strongly to individual bones.