As I said last week they only stole his bankai, not his shikai.
I guess it means it's just a shell of ice that has inside layers of ice and vacuums that switch each other off. Meaning that with each attack from the Quincy, a layer would be sacrificed, thus the wall would become smaller each time.
In the end I also don't really see the use of it. "Coincidently" a vacuum is very useful against heat/fire based attacks as it doesn't conduct any heat, however how could they have known that they would fight a fire user? Even though this tactic might make it appear like that the wall is thick, in the end it's still half empty. If they would have fought for example against that O-Overkill guy, he would probably have just smashed through it. Vacuums are useful, but it's not like he can control them. If there's one crack in the ice, the vacuum will be filled up with air.
So I don't really see that much potential in it that it could start replacing his bankai.
