A hop-up Airsoft device is a built-in or add-on device designed to increase the range of an Airsoft pellet gun by adding backspin to the pellets as they are fired. Backspin reduces air resistance across the top of the pellet allowing it to fly farther.
The backspin placed on the pellet is called a hop-up. That is how the device got its name. The hop-up device consists of a piece of rubber connected to a lever. The Airsoft user places pressure on the lever to activate the device. Moving the lever farther adds more backspin to the pellets. The rubber provides resistance to the top of the pellet which creates the backspin effect. High-quality devices can create back spins up to 120,000 revolutions per minute. With less air resistance across the top, the pellet drops more slowly than it would otherwise and moves farther without going any faster.
In order for the hop-up device to work properly, the Airsoft gun must be held in a proper position, horizontal to the ground. If the gun barrel is sideways, the hop-up device will add side spin instead of backspin and hurt the pellet's trajectory. If the hop-up lever is moved too far, the ball will drop early instead of traveling further.