Board Thread:Manga/@comment-34196931-20190808205129/@comment-27125793-20190812112628

I also have a theory about the Pure Titans, and it all boils down to this:

Awareness and will matter, on both Ymir's part as well as the Eldians'.

What I'm trying to say is that Ymir acts based on the will and knowledge of her Subjects. However, when she can't gauge their desires, she acts in a way that seems logical to her. Basically, when she senses that Titan fluid is inside the bodies of human Eldians, when Titan fluid should only be inside the bodies of Titans, she "corrects" the issue by turning them into Titans.

Shifters are exempt from this because they all hold a part of her soul, and because of that, she can determine their desires and will. It's been implied multiple times. For example, Falco said that a shifter won't heal if they don't have the will to live. In other words, Ymir was able to read Reiner's suicidal thoughts, and therefore didn't heal him.

Awareness also comes into play. The shifters powers are optimal only if they are aware of their power. For example, Eren never transformed during his tryining, despite doubtlessly receiving many injuries throughout, and having had concrete goals at least a few of those times... because he didn't know he had that power. He believed himself to be completely human, with regular human abilities, and the fact his wounds never released steam prior to his first transformation was probably a result of this too (the steam coming from his head wound is anime only, I believe). His arm and leg healing after being bitten off at Trost is also a clue. They didn't steam prior to him transforming and losing himself during that time, because he was still fully conscious and still held on to the aforementioned beliefs.

However, when he transformed the first time (which was probably a result of the fact he wanted to kill Titans 'by any means necessary' and all options, to his knowledge, had been exhausted), that awareness was lost and the only thing going through his mind was the will to live (so he could fulfill his revenge of slaughtering all the Titans). With those beliefs gone temporarily in that state, that roadblock they presented was gone too, and Ymir acted on that will to live by healing his injuries. That awareness was also the reason Eren transformed during the time he spent with the Levi Squad. Like in training, he had an injury, a goal, but unlike before, he knew of his Titan power, but he did NOT know how to use activate. However, Ymir acted on what he DID know and his then desire to transform (even if it was closer to the back of his mind at the moment), and with all the circumstances for the activation met... BOOM! Titan transformation!

Essentially, Titan powers are given and controlled by Ymir, who acts on the will and knowledge of their holders, but it also depends on her own awareness of those holders' will and knowledge.

That pretty much explains everything.