The FIS produced a video explaining some common violations of classic technique in a classic ski race. Here's a summary:
- In a corner, if there are tracks, you must stay in the track.
- If there is no track, it is fine to push off with the outside ski while the inside ski tracks around the corner. That is, you can't use the inside ski to skate.
- To switch tracks, it is OK to push (skate) out of the track, cross the the other track, and skate once to get back into the track.
- Switching tracks frequently is considered a violation, however.
- Herringboning, where the ski does not glide, is OK. Getting glide while herringboning is considered skating (aka - it's diagonal skate).
Here's the video: https://youtu.be/z07Z6AuBH6o