The other day I titled an article, 'French Curve' but I didn't really explain why in that post. What we were talking about that day in class was thinking about this particular set of exercises as being like using a French Curve.
That is, in the chains you know that there are points in space (relationships) that uke and tori should move through but it is not the points (techniques) that are important so much as the smooth, flowing, continuous curve through those points. You have to learn to move your body through an arc with uke without a lot of discontinuity. This is the same as a French curve - you plot a few points then find the template that matches those points most closely to help you draw your arc.
The moves in tegatana are small arcs. The moves in hanasu are French Curves. The techniques in nijusan or junana are the points that the French Curve helps you plot a course through.
We got a lot of mileage out of this analogy last class by working it with one-handed shihonage (that is releases #6 and #8). notice, that if you stand in place and hold uke's arm with your left hand and swing them through you describe an arc. Holding uke's same arm with your other arm you get a different arc through space. If you practice shihonage holding with both hands then you never have to develop this sensitivity to the precise arc that uke is moving through. That is why we practice two one-handed variants of shihonage - better opportunity to learn.