Never heard that term, but my brother shoots an 870 lefty. He uses the middle finger behind/under the trigger guard. I use the 2nd joint of my trigger finger to move the safety to the "fire" position and the middle finger behind/under the trigger guard to go back to "safe". We both use exactly the same technique, just backwards if that makes sense. I'd have no problems operating an 870 safety lefty.
This works fine on any gun with the CBS on the back of the triggerguard. Much harder if it is on the front. I find the Mossberg safety equaly difficult from either side and behind the triggerguard better for right or left handed users.