Board Thread:Naruto Discussions/@comment-1872728-20151110034635/@comment-734582-20151110111928

BakumatsuWarrior wrote: I hope someone clears this confusion for me. I would expect that her healing capacity exists in her body, but due to some weird mutation or whatever, does not manifests itself properly. So unlike say Hashirama who can heal without intervention, Karin has immense healing power but can not control or access it herself.

That's why biting works (It seems to -somehow- release her healing power) and why she biting herself works: because it's locked up in her body.