according to my leather craft tools book by Al Stohlman that is a Bisonette edger "and were used by the early saddle and harness makers" and they can be used in a pushing and pulling motion like Johnv474 has stated
I have achieved this by casing, stamping, applying super sheen as a resist to the stamping using a small fine tipped paintbrush, let it dry and then apply the stain/dye
Thanks Cappy, very easy to make. cut out a square of leather to the size you want it, that one i made was 9x9 inch i think. Then i folded the square in half so that it formed a triangle and punched a hole through the corners where the two sides met, I then unfolded it and refolded it so that it again formed a triangle for the other two sides that needed punching. all i did then was to put on the snaps. easy peasy lemon squeezy