Looks like your query about Okamoto's basic Roppokai exercise was lost in the dust, ... Okamoto's exercise seems to tune in on some aspects of aiki age--
The motion is a viscously slow version of what occurs if you use the center to throw the arms out with "backspin," basically, and they reach natural limits and whip back and rise naturally upward. The whole connection throughout the body is chain-like (in tension, anyway). You can do it with a chain, too, if you like -- or an uke, if you prefer.
It is easier to see the dynamic done larger and more energetically -- but easier to refine more slowly and mindfully, as with kokyu tanden ho exercises. Both work the same, saving only that the slow tends to revert to compression action instead of tension rather easily. That has a different flavor of dynamic, as well as passing through a neutral point that sacrifices kuzushi.and loses control of the unification with the uke.
Aikisage is the reverse, whipping downward.