My 1st Drilling - a Sempert & Krieghoff Blitz-Action - 16x16x8,57JR - 21-3/4" bbls

Although I've collected several Cape Guns over the years, I just purchased my first Drilling.

Its a - Sempert & Krieghoff Blitz-Action Drilling in 16ga,16ga, 8x57JR

I was drawn to this one because of the very short barrels, and its very good condition, with nice engraving. I've figured out the cocking and shooting sequence but have no idea as to the function of the little button ahead of the right trigger. I'm hoping someone here can help explain this feature.

As always, comments are welcome.