When doing long, straight hairlines, you can do it using the nib edge perpendicular. Often, when dragging out the curved hairlines, you aren't actually using the corner of the nib like a pen, what you're doing is dragging some of the wet ink from the bolder stroke. Every broad nib family has its quirks (Mitchell flexible but can be hard to start, Brause firm but quite oblique, Speedball smooth but thicker hairlines, etc.) -- that's what practice is for. Of course, you can always finish with a pointed pen like Hamid does here with what looks like a blue pumpkin (don't quote me on that).
This does not apply to using the Pilot Parallel, which will draw using the corner.