Onion Skin use on lesson

I understand what the onion skin does in other animation programs. Can someone explain how to better show the Onion skin frames. If this is not what your Onion skin tool is for, (as it is with Adobe products) please give a better description of its purpose and use in your lesson. I find the information on this topic to be missing at the moment.

Too what lesson(s) in Unit 3 would this apply most? I
I will be teaching Unit 3 for my third year and am already nervous about how little I know when students ask more advanced questions or are ready to add to what they already know.