What is Onion Skinning

Onion skin is not made from onion, at least the onion skin that is used in traditional hand-drawn animation.  Originally, onion skin refers to a very thin sheet of paper that animators can see-through the drawing on different paper sheets to ensure the consistency between frames.

To enable the onion skinning feature, please refer to this post, What is Onion Skinning and How to Use it.