How does a shape with a rectangular hole in it 'walk' down a square tube with ridges either side?

Makes me think of an old toy but I can't quite remember how it works. Is it the angle of the ridges or the weight of the shape, changing from side to side, that makes it work? Or maybe it's something that I've missed?How does a shape with a rectangular hole in it 'walk' down a square tube with ridges either side?
Yes, you set it rocking from side to side so it alternatively pivots on each ridge.


When it does that, the other side swings down slightly.





So it moves down the slope in a series of short steps.





I remember a toy horse that worked the same way.