Board Thread:Manga/@comment-29012123-20170511142229/@comment-45840661-20200709110209

I can agree with XSyruSX's theory, from my point of view the Attack Titan is like a human, although much stronger in physical strength and endurance, take an average Joe for example, this average Joe is failing his math test, or he is losing to a bully, but his determination and belief that he CAN pass the test or defeat the bully drives him to keep on trying, to strive for victory, and just like the Attack Titan the more motivation we have to do something, the better we do it, in the Attack Titan's case, well, it's only real motivation is fighting, so Eren focuses on that, Eren WANTS to keep on fighting no matter what, thus giving him more strength to continue on, no matter how much of a disadvantage he has he still somehow wins, or well comes close to winning, whoever just motivation, determination and pure willpower isn't gonna get someone very far, and so Eren uses his learned knowledge combined with his willpower to well, basically make himself stronger, so in my opinion it could possibly be the user's emotions that give them the strength to keep on going, thus why the Attack Titan holds it's name.

Short answer: Basically, in my opinion the special feat the Attack Titan has is growing strength dependant on how much motivation the user has, or it depends on what emotion they currently have present.