How does the flank attack's +1 die roll modifier work?

Question

Accepted Answer

The attacker names one space as the pinning space. Each other attacking space adds +1 to the flank die roll, unless that space connects by a solid line to another enemy-occupied space. Spaces with only an enemy fort do not count as occupied, dashed lines are ignored, and not every flank attack gets the bonus.

Paths of Glory

How does the flank attack's +1 die roll modifier work?