It's that little switch doohickey in the lower left corner of the receiver. It blocks the next shell in the mag from being loaded. The idea being that you can manually eject whatever shell is loaded and then manually load a shell with a different shot size if you get an uncontrollable urge.
Personally, I think it's the least used gadget in the history of shotgunning. I've never met anyone who actually used the thing. Ol' John Moses was a smart man but he reached a little too far on that one.