I'm growing broad beans for the first time this year and they seem quite
happy so far -- the blossoms are just beginning to open. My question is
about "pinching out" and blackfly. I've seen numerous references to
this but haven't been able to figure out what exactly is meant.

What I've been able to gather is that when the pods begin to appear, you
are supposed to pinch off the top couple inches of growth on the main
stem. Is this right? What exactly does this accomplish?