When you fire the shutter you should see the little pin from the body poke into the hole in the back.
The back should throw the tongue out to stop a double exposure.
The tongue should withdraw when you wind the back.
If the back is an earlier back it may be missing interlocks it will still work ok.
But you need the user manual for the camera as relying in hints from web not going to work.
google Mamiya RB pro user manual