There is not much blood because the pressure from the wood is actually sealing off blood vessels quite effectively. We all know to apply pressure to stop bleeding of any kind, right? That's one reason why you leave impaled foreign bodies in place for E.R. to remove in a hospital setting that can act immediately to prevent more bolood loss and trauma to the victim.
Had to have fell on that from some height, huh? I didn't see any other bruises from like...a car accident...I dunno. Wish I knew the story behind it, though.