I recently received one of Allen Lowrie's mystery tub-o-tubers and
included was a little diagram showing what happens when the tuber goes
dormant. From what I could gather, the tuber is not completely formed
when the above gound portion of the plant dies back. It takes about a
month to finish formation of the tuber after the plant has gone dormant.
Of course, I have limited experience with tubers, so this is all 2nd hand
advice.
Davin