Onion skins show you the position of the figures in previous or subsequent frames when editing a frame. They are only shown in the frame editor and not when playing or exporting an animation. By being able to see the relative position of figures in neighbouring frames it makes it easy to judge the amount of movement that should be applied to a figure to create realistic and fluid movements. In traditional hand drawn animation, this is often done using tracing paper or light boxes.
Background Onion Skin
The background onion skin is similar to the onion skins in Pivot v2, except that they are in colour and semi-transparent. They show all the figures from the previous frame only and are displayed behind all the figures in the frame editing area, but in-front of the background image. This is so that they don't have to be redrawn every time a figure is moved and so makes moving figures faster.
Selected Figure Onion Skins
The selected figure onion skins show onion skins from multiple previous frames for the selected figure only. If multiple figures are selected then they are not shown. These onion skins are shown just behind the selected figure and so can normally be seen even when the editing area is filled with figures. They are shown in colour and are semi-transparent with the amount of transparency increasing the further away that frame is from the frame being edited.
A selected figure and its onion skins showing its position in 4 previous frames
The number of selected figure onion skins can be changed from 0-9 in the options window. There is also a check-box that enables showing onion skins of subsequent frames when editing a frame in the middle of an animation (Show Ahead). Displaying this kind of onion skin can slow down the movement of figures in the frame editing area, so if it becomes to slow try setting the number of onion skins to zero. The number of onion skins is set to zero by default for this reason.
The selected figure onion skins work by giving each figure an ID number that associates it with the same figure in other frames. When you add a new figure to the editing area or duplicate a figure, the new figures will be given a new unique ID. When you add frames to the time-line, the figures keep their ID's so that the same figure has the same ID across all the frames that it appears in. This means that if you delete and then add a figure then you won't see these onion skins for it since it will then have a new ID.
After loading a Pivot v2 or v3 animation, you won't see these onion skins until you start adding new frames, because figures will have a unique ID in all frames. This is because Pivot v2 and v3 didn't support this kind of onion skin and so the PIV file doesn't store the figure IDs.