Quote (MynameFailed)
Chaper 8 page 17 and 3 pages forward. I dno, might have been bad explanation from me :\
Sorry for slowpoke reply.
Tatsuya's ability states that it loaded backup Eidos into him. Eidos is the phenomenon that magic alters (such taking a rock at rest, and using magic to increase its veolicty. The rock's existence (Eidos) changes from one that is not moving to one that is. In Tatsuya's case, he is able to store a back-up of his own Eidos. When he took severe damage, he automatically exchanged his present, damaged body with the back-up copy.
How he is able to perform such magic is unclear, but it does coincide with his ability to perceive magical structures.