bit breaker

n. [Drilling]

A special tool used by the rig crew to prevent the drill bit from turning while the bit sub on top of it is tightened or loosened. Bits have noncylindrical shapes, so the conventional wrenches used by the rig crew to tighten cylindrical shapes like pipes do not fit the bits properly. In addition, some bits, such as PDC bits, have a wide range of unusual and asymmetric shapes or profiles. The bit breaker must match the bit profile or the bit may be ruined before ever being used. ........

                                                  source: http://www.glossary.oilfield.slb.com