"When Aizen gained possession of Urahara's Hōgyoku, the seal the latter had placed on it was broken, but the orb still needed time to awaken from its dormancy. However, Aizen discovered a way to temporarily bypass this awakening period. He found the Hōgyoku could be fully awakened, if only for an instant, by allowing it to momentarily fuse with an individual who possesses at least twice the Reiryoku of a captain-level Shinigami. Aizen himself possesses such a level of reiryoku, and therefore uses the Hōgyoku to enhance his Arrancar army." - cited from Hōgyoku