no one has probably thought about it for awhile..
but those rings, i just realized they are probably the same material pein is using on those bodies to control them.
its how he could speak to all akatsuki, so i think its very likely the same stuff. whenever jiraiya or kakashi were stabbed he could had control.
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i think pein plans on controlling akatsuki at some point, or prevent them from doing something he doesnt want them to do.

it also explains why he seems to listen to madara. madara wears one of those rings so if pein wants to he can control him all the same.

its like a hidden trump card on all of the other ninja in akatsuki, which are some of the only ninja that can fight at his level..
what better way to keep them in control and also give yourself new more powerful bodies to replace some of the other 6 when need be.
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i could get alot more in depth but ill let someone else do that..
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i just think thats what they are, it makes the most sense.