Well, maybe related, abstractly. Why would the potentials exist only at the points of reversal?
Technically, because a dynamic is no longer a potential. That's what potential means. In-yo operates in continuously complementary potential/dynamic. When a yo potential becomes dynamic -- it loses its potential, but the yin potential increases in inverse proportion until the yo dynamic is either exhausted or stopped - at which time yin potential is instantly and fully available. If one dynamic reaches any limit (i.e. -is stopped at any point) it simply returns instantly to the release the opposite potential into its dynamic, the same as when you reach the limit of the body's own extension in various aiki taiso. The actual spatial extension of limbs or the action is irrelevant -- as the pulse exercise shows.
So potential does not only exist at the end of the extension of the various aiki taiso exercises -- but that is where the potential can usually be felt FELT as a pure potential at a passive point
conscious intent (or trained action) to create it -- Also, the dynamic is naturally bled out to eliminate the noise of action from the perception of the potential in the pure structural condition at a natural dynamic limit.
The aiki taiso gives a reference point one does not have guess at as much for the bodily sensation to know what to aim for in training. That is, at least my perspective of it. Admittedly, they are done badly for this purpose in many, many places.
It is a reverberatory (oscillatory) conservation of momentum -- (yamabiko) -- even when not moving, that potential can be poised and held in the structure -- like this pulse example you prompted. It has long period undulatory aspects (the "demon snake" funetori, udefuri, (and I would say -- all conventional waza). It also has short period vibratory or pulse aspects (the "spirit of bees," furitama, tekubifuri, pulse strikes like the example at hand).