the movement is from the shoulder in such a way so that the forearm remains completely 90 degrees to the keyboard at all times.
not quite..
this would not be possible while playing at either extreme of the keyboard right? such motion uses the upper arm lifted out sideways holding the forarms, and is very tiresome. The body should remain inline with itself, but it can at the appropriate times be at angles with the keyboard.
EDIT: I know that may be confusing, I'm trying to find an appropriate photo via google but am struggling a little.
EDIT 2: Attached image - incredibly dodge explanation, best I can do right now.
Image shows drawn LH in 2 different positions, one that would apply to the middle of the keyboard, and one that would in theory apply to the lower end. Roughly.