There has been surprisingly little reference made to the process of "superimposition" in 2D animation in the past. Inexcusable to my mind as the really good inbetweening of widely spaced keys is not really possible without using the technique. It's also not a thing featured in most modern hand-drawn 2D animation digital software - which is very regretful for the same reasons as already mentioned. So, to clarify for students of traditional 2D animation, I made this quick video to explain the process. I hope it's useful?