This article, Fuinjutsu: Seal of Absorption is property of Almighty. The darkness watches, insanity stirs, touch my stuff, and death will be yours. You have been warned.
Aizen had created a seal for him to absorb ninjutsu attacks. He places it on which ever part of his body that he wants it to absorb the ninjutsu to. When he has the seal activate on him, he can absorb only the ninjutsu he wants. But the limit of ninjutsu he can hold at a time is 5. He can release it when ever he wants, so that it shoots the attack back at the opponent. And what ever order he wants. This would cost a lot f chakra, but Aizen has reduced chakra cost, and a massive amount of reserves.
