A right handed screw is driven in by rotating it clockwise. A left handed screw is driven in by rotating it anti-clockwise. What does the rotation of a clock have to do with handedness?
The truth is, there is nothing about the screw that gives it the designation of right handed or left handed, in fact, this designation comes from our anatomy and physiology.
When you hold your arm straight out and rotate your hand so that the palm is up, this is called a supine position and the act of rotating your hand is called supination. When you rotate your hand so that your palm faces down, this is called a pronate position and the acto of rotating your hand is called pronation. Awesome, now you can get that question right at your local pubs trivia night! But what does it have to do with the handedness of a screw?
Simple, the muscle that supinates your hand when your arm is stretched straight out is called the Supinator. The muscles that pronates your hand are called the Pronator Teres and Pronator Quadratus. These muscles are all very small and weak, making this motion a rather weak movement. This is why it is hard to get any leverage on a screwdriver with your arm outstretched. If you bend your arm, the biceps brachii comes into play and can help supinate your hand with a lot of force, driving the screw all the way down!
As you may have noticed, your hands supinate in opposite directions, your right hand supinates in a clockwise direction while your left hand supinates in an anti-clockwise direction. This means that you can drive a right handed screw with your right hand using your supinator and bicep, while your left hand would only be able to use the pronator muscles. Hence, a right handed screw can be driven in with the greatest strength when using your right hand.
This little mnemonic helped me learn supination and pronation while in dental school:
You hold the soup, and you "pro" it out.
To hold the soup, your hand needs to be supine. Turning your hand into a pronated position would throw out the soup.
The take home message is when you need to put in and take out a bunch of screws that are all right handed. Drive them in with your right hand but back them out with your left hand. This will make sure your strong biceps do all the work and your forearms will not get fatigued.