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You need the active key for posting authority changes. It is like adding a new posting key so we don't have to know your old key.

Oké, so this 2nd posting key makes it possible to know if I voted on a post myself or if the Bot did the posting?

How easy/hard is it to retract posting authority?

Yes, exactly. And it is as easy as adding it. Steemconnect even has a gui for it: https://v2.steemconnect.com/apps/authorized

Last question, when/why do i fill in these things?:

They are currently disabled and I will probably grey them out tomorrow, sorry for the confusion.