
I hope this picture clarify what a partial collar looks like, have to borrow a better iPhone camera to do this shot, imagine the coin blank didn't completely fit into the chamber to complete the reeding, the incomplete area would be partially smooth, and the finish area will have the "steps" you normally see in a complete coin, this depends on how the coin seated.
If you live near a local coin shop, and by chance they have error coins of this type, once you see it in 3 dimension, you will understand what a partial collar is if the picture still don't clear it.