Well, I finally got the time to do some checking.
From "Total Aikido, the Master Course", by Gozo Shioda [Kodansha, 1996]:
"Fix your power," "the energy is fixed." Lead uke's power so that it reaches its full extent of movement, at which stage uke loses his ability to pull back or withdraw. By fixing the energy, uke is not able to let go of the hand he has grasped.
In "Dynamic Aikido", by Gozo Shioda [Kodansha, 1977] on page 36 and again on 38, Shumatsu Dosa is refered to as "Fixing Movement" as opposed to the more commonly used "Finishing Movement" or "After Class Exercise" noted in the sources Steven sited above.
While possibly a mistake in translation in the one source, I think the definition given of "fixing" in "Total Aikido" is very telling; I believe it bears some serious thought.