I think he meant, flat bow hair. Lots of people see professional violinists tilt the bow hair at the frog, and then imitate that by tilting the bow at all points in the bow, which is not correct. Try to keep bow hair flat at all times, unless playing at the frog. When you play at the frog, you have to raise the bow arm slightly, which tilts the bow slightly away from you, in order to not bump against the bottom bout. This usually is only a problem when playing on E and A. Still, we tilt the bow at the frog because the hand is heavy on the bow there and very often you are playing spiccato or off the string.