Garry, the mfgrs might have intended that function, but I’ve never met a roofer who used the under blade or corner on asphalt shingles. They’re quite abrasive, and two squares would radius the corner so as to be about useless. Felt paper, yes - I have seen cut that way a few times, especially during tearoff. But every roofer carries a retractable utility knife, which works much better. (Old coots sometimes carry a nonretractable one, like a Stanley 199 or similar. If they’re real old and coot-y, they have a little stone to touch up the blades. DAMHIKT.)
Parker