Strafing is the act of walking sideways when under fire to reduce hits taken.
Original Author: Pimgd
When: Whilst taking fire.
What: Moving whilst returning fire/reloading.
Why: The enemy has you in their sights and is pumping you full of lead. Moving out of their sights prevents bodily harm and increases your lifespan. Taking active measures to keep out of their sights is even better in that regard.
How: When someone attacks you, start moving before you rotate to the source of attack. When you are reloading and there is no cover nearby, sprint left to right and right to left to minimize the hits you take.
Bonus: When your opponent has a weapon that is not suited for close combat (anything other than a knife or shotgun), and you're close by, consider running circles around them instead (see Circling). Some players don't have enough mousing space or have their mouse speed set too low to do a 360 rotate without lifting their mouse. That way you can further reduce the hits you take.
How to practice: Get a friend to shoot you in the legs at a distance. Your task is to sprint around. Try starting in warehouse, the shooter with his back to the wall, and you with your back to the other wall. The shooter moves forward after each round if you survive a magazine.