Decision. Course-up turns the chart under the boat — but below about a knot the rotation freezes at the last good heading and an amber badge says so. North-up stays the default.
Because. This plotter has no compass. Its only sense of direction is course over ground from GPS, and course over ground at rest is the direction of the noise: roughly thirty degrees of error at half a knot, eight at two. A naive course-up would spin hardest exactly when the chart is most stared at — at anchor, on a mooring, coming alongside. Where the boat has its own compass, that one wins: it works at zero knots, and it points where she points rather than where she is going.
Instead of. Smoothing the heading until it looks calm. That hides the same lie behind a nicer animation.