In traditional animation Onion skinning is a technique where you can see several frames ahead and behind your current frame at the same time to check on the difference in frames ahead and behind while you draw the current frame. It was acheived by stacking the drawings on a pegboard on a light table. It was common when you actually had to draw each frame by hand on a piece of paper on a light table/animation disk.
I think what James wants to be able to do is see translucent frames ahead and behind the current frame as a reference tool so that you have a greater feel for where the relevant motion to the junk matte takes place on screen.
my suggestion: add a few temporary layers while you make the junk matte and then delete them when youre done. You could lay a whole bunch of layers at low transparency and offset by a frame to the left, or maybe someone can suggest a plugin that does an ‘echo’ or trails effect.