This Pose Brush origin mode simulates an FK deformation in the entire
model when clicking on the face sets, as they were controls of a fully
rigged character. Combined with the previous Face Sets modes that allow
creating IK chains, the pose brush should now be able to simulate most
of the common rigs deformations.

The UI name of this option and the previous one can be changed to make
more clear how they are intended to be used. Once brush management is
there, this will be presented to the users as a Pose Face Sets IK and
Pose Face Sets FK brushes, so the naming of the option will be less
relevant.