[i] I think there are a couple of different scenarios regarding the hop. One, as Jerry suggests, is a voluntary hop that one does when one knows that an opponent or push hands partner has you at a disadvantage, and you're about to be launched. It's a way of maintaining your frame, and quickly re-establishing your equilibrium... .
I do this occasionally when I get hit with a lot of energy that I know will not be pleasant to deal with - just another tool in my ukemi tool box. Of course if the focus of the training is to receive the energy and work with it in some manner, than that is different - but if I am attacking and someone gets my energy coming back at me hard, I will do the voluntary hop back to help dissipate the effect - much easier on my old body