You can only start from where you are. Sorry for stating the obvious, but it may be useful to remind ourselves of certain constraints of reality.
The morote dori grip starts at jodan level, and uke's goal is to lower tori's arm in order to control it optimally. You can see this in the video:
In this dynamic of uke's grab, tori has two options:
1- He acts at the start of the grab, which is at the top (ue), at the moment when uke tries to close his hands around his forearm.
2- He was unable to act at the start of the grab, and uke had time to grab his arm with both hands and bring it down (shita). In this situation, tori can obviously only act from the bottom.
In case 1, the spiral he describes is necessarily executed from top to bottom.
In case 2, the spiral he describes is necessarily executed from the bottom to the top.
It cannot be otherwise: a downward spiral can only start at the top, and an upward spiral can only start at the bottom.
What must not be done under any circumstances is a half-spiral upwards followed by a half-spiral downwards, or vice versa: in Aikido, once a spiral has been started, it must be completed. All Aikido movements are expressed by two symmetrical and complementary spirals. It is this characteristic that makes it analogous to life, whose DNA - present in all species - is structured by two symmetrical helices wound around a common axis.
When the movement of Aikido is performed correctly, it is connected to DNA, to the great cosmic serpent that unites all living beings under the sun. Aikido movement is a symbol of life, reproducing its model.
