Onion skinning is a 2D animation technique. In the making of traditional animations, artists draw on very thin papers and place the papers on a light source. By doing so, artists can see through the frames, and compare the keyframe and the in-between frames.
In Animation Desk, you can open the onion skinning feature in the settings. Be sure you also turn on the lightbox. There are different options to see through the frames.
Opacity Onion Skin
The opacity onion skin is available in the Basic plan. It shows the frame order with levels of opacity.
Color-Coded Onion Skin
The color-coded onion skin shows the frame after the current frame with red shades, and the frame before the current frame with gree shades. Color-coded onion skin is available in Creativity 365 and Animation Desk Pro.