User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-31764731-20180326095200/@comment-86.153.134.144-20180329014327

And reading Vol 12 in more detail Issei Soul only stayed in an alive state because his soul was blessed by Chichigami and the past possesors use the energy of the trapped soul fragments to move that blessed soul into the booster gear until his new body was ready.