Did this work with your student. I'm having trouble understanding exactly what Chang is saying. I can do very rapid 5,1 parallel sets on the octaves, and slightly less smooth 1,5's. These without any arm rotation. The result has a "pause" in between, definitely doesn't sound like a tremolo, kind of 5,1,,5,1,,5,1 with the first slightly louder than the second.
Alternatively, I can do arm rotation. Sounds pretty good, but I can't get any speed up, at least without it not sounding terrible.
What I can't work out from Chang's text is to somehow combine the up-down of the parallel sets with the arm-rotation.
Did you find a good way to explain how to do this combination? If so, please share :-)
Thanks, John