Hop-up & Bernoulli's Principle

Bernoulli's principle as applied to an airsoft pellet is as follows. As a spinless spherical pellet flies along its trajectory through the air, the pressures on all sides of the pellet are equal because the air is traveling the same velocity relative to the surface of the pellet. If a spin is applied to the pellet about an axis perpendicular to the velocity vector (e.g, a backspin) the air will be rushing faster (relative to the pellet surface) on the side that is spinning away from the velocity vector and slower on the side that is spinning towards the velocity vector. Bernoulli's principle says this difference in fluid (air) velocity implies a difference in pressures, which is a force that will cause the pellet to move in a direction perpendicular to the velocity vector.

Airsoft hop-up devices apply a backspin to the pellet so that the pressure force acts on the pellet opposite the direction that gravity is pulling it. This causes the pellet to fall less over a given distance than it would without the spin applied to it.

In airsoft guns this is often implemented as a rubber piece at the rear of the barrel that is thicker at the top of the barrel than the bottom. As the pellet moves past this piece it tends to roll, inducing a backspin. This is usually adjustable so that the effect can be tuned.