There was an ONL match a while back, or maybe it was 80hp, I can't remember exactly... I can't even remember confidently enough who it was, although i'm like 75% sure it was Dario...
Anyway, he explained about this one way girder thing, and remember either him or Mablak explaining about using the Windows DPI feature so you can actually move the mouse 1 pixel at a time using the numpad or something very easily in order to place one-way girders that way.
Maybe one of those guys can explain it better.