Flexspline: thin, toothed, bent by the ellipseOne flexspline tooth: the outputCircular spline: rigid, two more teethThe elliptical wave generator bends the flexspline so its teeth mesh with the rigid ring at two places, opposite each other. The ring has two more teeth than the flexspline, so each turn of the ellipse walks the flexspline back by two teeth. Here that's 50 teeth over 2, 25:1. Real ones use finer teeth, 100 to 160 of them, for 50:1 to 160:1 in one stage. Because many teeth are always partly engaged and the flexspline is preloaded into the ring, there is essentially no backlash. The cost is a part that flexes every cycle, some springiness, and less efficiency than a planetary.