Retroplayer:
oh... the follower is long enough that it touches both sides even at the crossing points so it stays on the track it is on. And the tapers on the end of the follower are designed to allow it to flip angle at the ends. The shape of the follower is actually the most important part of the mechanism.
I imagine something like that would be required, but I'm struggling to follow how that would enable you to adjust the end points i.e. have a spool that has a crossing point that can optionally be 'blocked off' to make the follower reverse at that point. If the shape of the groove and follower make it possible for the follower to reverse at that point, what prevents it from reversing even when the crossing is not 'blocked off' iyswim? It seems to me that for this to work, the turning points at the ends need to be a different shape to the crossing points in the middle. Maybe there's a devious way round that, but I can't figure it.