Effect is that you get increased dodge chance against every attack from direction, relatively which you have moved diagonally (or perpendicularly). This works only for next turn, for example: you moved, even if enemy wasn't firing in his 1-st turn after yours, you do not get any bonus in his 2-nd turn if you won't move again.
I don't know the numbers, but practically it's significant, it really makes retreating (and approaching) much easier. And, yes, Hellrunner is a very good trait if you want to dodge most of the projectiles.