A ratcheting screwdriver has a mechanism in its handle that allows the user, when screwing or unscrewing, to reposition his hand for another turn without having to remove the head of the screwdriver from the screw’s slot. When using a ratcheting screwdriver, there’s no need to release the pressure on the screw or to remove the screwdriver’s head from the screw’s slot. Instead, the user just turns his wrist counterclockwise.
When driving a screw with a manual screwdriver, one exerts pressure on the screw while turning the screwdriver clockwise. The amount the screwdriver can be rotated, though, is limited by the user’s range of wrist motion. In many cases, only half a turn can be executed before the user must reposition his wrist. When repositioning the wrist, the pressure on the screw cannot be maintained; this may pose a problem if the screw hasn’t yet been securely seated. Even if the screw is seated and isn't in any danger of falling out with the release of pressure, time is lost if the user must pull the screwdriver’s head out of the screw’s slot and then reposition it for the next half-turn drive of the screw.
With a ratcheting screwdriver, the screwdriver’s handle turns freely with a “click-click-click” of the ratchet re-engaging, but the screwdriver’s shaft and bit remain steady, locked in the screw’s slot. The screw must be somewhat set for the ratchet to work, though. If it isn’t deep enough for there to be resistance to a counterclockwise turn, the ratchet won’t engage properly.
A ratcheting screwdriver incorporates a ratchet inside its handle, with a gear wheel and two pawls. A ratcheting screwdriver has a switch in its handle with which the user can choose which direction will be the restricted one. Thus, if driving a screw, clockwise is the restricted direction; that is, the handle won't be able to turn clockwise independently of the shaft. If the handle’s turned counterclockwise, though, the handle will turn, but the shaft won’t. If the screw is being backed out of the hole, the switch is reversed and counterclockwise is the restricted direction.
Ratcheting screwdrivers are usually sold with a variety of bits so they can be used with many different types of screws. Some also have pivoting handles which, when engaged, give the user a boost in the torque that can be applied. A recent development is a ratcheting screwdriver with a flexible shaft. This is a useful tool for screws in hard-to-access locations, but require two hands for effective operation; the flexible shaft is actually a drawback in routine applications.
These screwdrivers are best suited to working in wood. When driving screws into sheetrock, the special requirements associated with driving each screw, as well as the sheer number of screws required for proper installation, necessitate a power tool like a drill with sheetrock bits or a sheetrock screwdriver. Likewise, when driving screws into machined holes, such as those on junction boxes, there’s usually not enough friction on the screw to be able to use the ratchet function properly.