I've probably said this before, in another thread, but anyway: one problem with the cupped-hand pose is that although it may enhance the sound for the singer, it can reduce the quality of sound and the clarity of the words for the listener. Some members of the audience may have the protruding elbow blocking the path of the sound from the mouth, and also blocking the view of the singer's face, both of which make it harder to hear clearly and understand the song. Basically, the arm and hand muffle the sound in one direction, and what the singer gets is a false idea of how he/she sounds to the listeners.

I really don't understand why harmony singers do it either. I've sung lots of harmonies in choirs, duos, and small groups. I don't want to hear the balance distorted by funnelling my own voice back to me - for me it's just as important to hear what the others are singing, so that my singing will tune in and blend with theirs.

Marje