Below is an excerpt from one of the old Italian books on singing. It contains a teaching tip that is useful for any voice teacher in any genre.
Practical Reflections on the Figurative Art of Singing (1776) by Giambattista Mancini (trans. Pietro Buzzi), p. 97:
The easiest way, and the one by which I have had good results, is to give the student the evidence of his error, and to this end, he must faithfully reproduce himself the defect of the student. He will then know plainly the error of singing in the nose, or in the throat, or with crude voice and dragging. Hearing the teacher reproducing his own faults, and seeing him making an exact caricature of himself, enlightens him. He will then remark, confess and condemn those errors, which he would never have remarked, confessed nor condemned in himself.
This relates to my previous post about the difficulty in listening to oneself objectively. Hearing oneself imitated by another can help one to confront faults immediately, with fewer filters and less confusion than listening to oneself during the act or on a recording. Of course, in order to do this well, a teacher needs a flexible voice and a keen ear.