This feature is experimental.
It may not work as intended and/or change a lot in the near future!
Currently, hitting the ghost icon will show a list of how many frames you want to show as onion skins. To disable it, just choose none.
Attention!!! Attention!!! If you don't use a transparent background, set up your secondary color to be the color of your background, otherwise the onion skin won't be able to tell which color is the color of your background, thus, it won't be able to cut the background out, making impossible to render previous frames behind the current frame.
Well, just sort your primary and secondary colors and you'll be fine!