[bartbutler]

It's a key escrow system. The admins hold a copy of the user's key, and promoting a new admin involves giving access to this key.

[ComodoHacker]

So where is this copy stored?

[ThePhysicist]

Interesting! You could probably do it by using the key of an existing admin to decrypt the escrow key and then reencrypt it with the public key of the new admin, all client-side.