I don't think Shauriatas controls his motion, sci.
The excerpt seems pretty clear that Shauriatas' soul is simply being
deflected from the Outside.
It also offers what seems the most concise explanation for proxies, thus far (minus Seswatha's dream of the Coffers):
“But souls are exceedingly complicated,” he continued. “Far more so than the crude sorceries used to trap them. The intricacies of identity are always sheared away. Memory. Faculty. Character. These are cast into the pit... Only the most base urges survive in proxies.”
Which was what made them such useful slaves.
“So to have your soul caught...” She trailed, frowning.
“Is to be twice-damned...” he said, trailing at the behest of a queer reluctance. Few understood the monstrosity of sorcery better than he. “To have your hungers enslaved in the World, while your thoughts are tormented in the Outside.”
Synthese, on the other hand, seem living projections of soul onto matter. Both Animata and Synthese are facilitated by sorcery...
Animata require other previously ensoulled vessels, whereas, the Synthese (allegedly) requires a
concert of sorcerers. I write allegedly because I'm paraphrasing Kellhus and he was speaking on Compulsion from Golgotterath to Esmenet.
Perhaps, that sorcerous trifecta is the key to this metaphor. Hrm.