Screwdriver bits are typically made up of a shaft and a tip. A screwdriver bit's shaft is typically hexagonal in shape, with six flat sides. This shaft is fitted into the end of a screwdriver that has a bit holder. The shaft's shape prevents the bit from rotating within the bit holder. The tip end of a screwdriver bit may have many shapes.The bit's head is designed to fit into the head of a screw. As the screwdriver is spun, the bit rotates as well. This is what causes the screw to turn. Head style: Torx Security T40 Length: 25mm PTI T40 x 25mm 1/4" Hex Tamper Proof Screwdriver Bit An alternative to regular torx bits are tamperproof torx bits. They are not solid, but instead have a hole in the middle for security screws, preventing the use of conventional torx bits. They come in sizes from T1 to T20 and are typically put on objects that people don't want you to have access to.