Wera's Bi Torsion Extra Tough bit range is specially designed for fastening into Metal. The extra durability combined with a softer Bi Torsion zone allows the bit to absorb the sudden increase in torque applied when a screw comes to a stop in Metal. The Bi Torsion zone avoids premature bit breakage & enhances the service life of the bits. The Bi Torsion system means that the bit has been precision hardened to allow the metal to flex slightly in controlled areas & absorb the stresses applied by a power tool. Extremely hard wearing, premium quality. (Use the Wera Impaktor System for unbeatable performance in high voltage impact drivers).