It was Mac that lit that light over my head. He was demonstrating a simple in-the-air mend, like they used to ask in the FFF test, where you make an out-and-back or in-and-back move after loop formation. He briefly added that accentuating one move and relaxing the other would impart a momentary change in momentum which would result in a curve.
Calling it an impulse or out-of-line thrust was a product of my mind... probably due to too many drugs in my youth? But, I think, what is happening is a change in momentum, which sets up a wave, in this case horizontal, and that shows up as a curve when the cast is settled onto the water.
It also made me wonder if it would not be an easier way to produce both right and left curves from a vertical rod plane. I played with that very briefly and it appears to be the case.
For the fishing I do, being able to perform good curved presentation is the primary difference between an eat and a spook.