I had to google this. Sounds interesting. Don't have a Bernina so can't
tell you how it works but here's a video for those wondering.
https://www.youtube.com/watch?v=3plKN6vw2ys
Here's another thread about this foot (including another video).
Looks like it is just for outline. It is not free motion quilting.
You can't move the fabric like for FMQ.
Watch the videos and you will understand how it works.
Bernina leather rolling foot #55