I don't think I've seen this asked, so what happens if you deviate from the course extracted from Maps on Us?

Your current-location pointer seems to drive around in the blank space outside of the MapsOnUs route, and you see a "rubber band" line indicating the direction to the next expected waypoint in the MapsOnUs route.

The software seems to do a decent job of skipping ahead/back to the correct waypoint once you've re-acquired the track and you're back on the right road. If it doesn't, a quick twist of the knob will select the appropriate waypoint and you can see the rubberband updating on the screen as you do. Rather slick, actually.