Left hand drill bits are drill bits where the thread of the drill is reversed from a normal drill bit and they cut when the drill is spinning anti-clockwise.

Left hand drill bits can be used to help extract screws which have become stuck and the head is so damaged that it is of no use. Due to the fact that the left hand drill bit is essentially trying to loosen the screw as the bit moves into the screw it removes it providing the bit "grabs" the damaged fixing successfully.