In Kaza’s interlude she describes the process of soul casting as “picking at [the stone’s] memories,” and notes: “She nearly gave in herself. How wonderful would it be to no longer fear? To soar into infinity on the air?” (Oathbringer Hardcover Page 561). To me this indicates that for some reason the fabrial machine, while directed by her mind, is also acting upon her spirit. Her will for the stone to change in the passage is also noted to be reinforced by another: “commanding and powerful, attracted by her request for aid.” Shallan does not note any reinforcing will like this either of the times she soulcasts, which makes me think that it is this introduction of a third party, controlled in a relatively crude manner, that makes the use of a soulcaster fabrial machine have this effect on a person.