The plug would be a dummy element. Instead of a cable it could have a chain on it. The plug inserted open that particular circuit stopping flow to the relay. The plug would be open, as in no connections. It physically holds the pathway open. When all plugs are in, no juice to the relay coil. Any plug pulled sends juice.
When using safety relays, instead of daisy chain hot circuits such as classic e-stops, this the norm. You are correct about the old style use.
When using safety relays, instead of daisy chain hot circuits such as classic e-stops, this the norm. You are correct about the old style use.