Ben,

The Darne design dates back to the paper shotshell era and was designed to help in loading swollen shells. Not everyone likes the way they look, and, for a long time they were derided because they were "only" a boxlock, but if you handle them, you would realise that the French had a point, a very good one. I can't remember which shotgunning authority called them "greased lightning" but the description is very apt, if somewhat unusual.

Try handling one of them and you'll know exactly what the gentleman meant.

Good hunting!