1. Go to SU settings and set up a pin.
2. Go to an app which uses su but hasn't the right to use it.
3. You are asked for the pin. Deny the request but make sure you don't save your decision.
4. Now go to SU settings and set the auto approval to accept requests. (No pin required!)
5. Go back to the app and try to use it again.
6. Now you have granted the su rights to the app without knowing the pin!
Hope this helps understanding the problem.