Zum Drehvorgang der Darboux-Achse einer Raumkurve

In one of his papers J. Hartl says that he decomposes the Darboux rotation for the Frenet frame of a space curve into two simultaneous rotations. What he does, however, is to decompose the rotation for the Darboux axis of the space curve into two simultaneous rotations. In the following approach this rotation for the Darboux axis is being described from a more general aspect.