When working on a traditional animation desk, you will often take the drawing you are working on off the peg bar (the clips that keep all of your pieces of paper perfectly lined up) and move it around allowing you to trace a part of the previous drawing that is moving across the screen.
I'm not sure how feasible it would be, but it would be really useful to have an Off-Peg function that would allow you you move (and rotate?) the whole canvas around on the current frame whilst the canvas all the other frames remained in position. Switching the Off-Peg function off again would snap the canvas back into position.
You can sort of replicate this function by selecting and moving parts of the underlying drawing around and then undoing the movement once you have finished tracing, but it's a bit tricky and doesn't really work in the way a dedicated Off-Peg function would.