Although it sounds like a magazine cutoff is engaged - I seem to remember my 20ga AL-48 working like that.
IIRC, the feature allows a shooter to switch the loads chambered, for a particular shot, w/o having to unload/reload the entire mag.
Like, say you're bird hunting, and see a deer coming by.
If it's legal, you could exchange the birdshot load quietly for a slug or buckshot to make your play.
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