Every airsoft player should know about hop-up, and most players do. However, if you do not know, here is a simple definition. Hop-up is basically the trajectory of an airsoft pellet, and when an airsoft gun has adjustable hop-up, that means you can adjust the trajectory. Hop-up is also the theory of backspin, in which spin, usually up-spin, is placed on the pellet as it grinds along the top of the inside of the barrel. So, the friction creates the spin, and then that spin offsets things such as gravity and air resistance, allowing the BB to travel further. However, changing the hop-up to increase accuracy only increases the vertical accuracy, but not the horizontal accuracy.
Hop-up devices are used on airsoft devices to apply more backspin to pellets. As stated, the backspin allows pellets to fly farther and fall less. Since the pellets are light and wind ultimately affects the trajectory, pellets can still fly off to the left or right. Hop-up can be adjusted to fit the weight of the BB, or just to do anything the user wishes.
A main principle that relates to hop-up is Bernoulli's Principle. Bernoulli's Principle states that "in an ideal fluid (includes air), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in the fluid's gravitational potential energy." How does this apply to airsoft? Well, if you read the definition, pressure is inversely proportional to velocity, in that lower pressure equals higher velocity. A spinless pellet will have equal pressure all around it, so the only thing determining its distance is the FPS. The next part is quite complicated, but simply, backspin creates a difference in pressure on opposite sides of the BB, and this difference makes the pellet go further. Another interesting thing is the Magnus effect. Pellets have thin layers of non-moving air on top of them, and when spin is applied, the air pushes off the pellet at a downward angle, and by Newton's law, then pushes the pellet up, giving it lift. In a non-spinning ball, the air would all go off at the same angle, so there would be no difference in trajectory due to that.
As you can see, by adding spin to pellets, the accuracy of an airsoft gun can be greatly increased. That is why it is wise to purchase an airsoft gun with an adjustable hop-up, so you have greater control over where your pellets end up. Knowing how things function (in this case, hop-up) can give you greater confidence, since you understand how the hop-up device and hop-up in general works. Of course, you can also impress people with your intelligence, which is always an added bonus of knowledge.
