Hop-up Explained
Hop-up's main function is to increase the range of the BBs travel, making for longer shots. However, not all airsoft guns have a hop-up mechanism (check the make and model of your gun).
How it works
The hop-up mechanism on an airsoft gun imparts backspin on the
BB. An adjustable piece of rubber provides resistance on the upper
part of the BB when fired, causing backspin.
Adjusting
The hop-up lever adjusts the amount of backspin you impart on
the BB -- from normal (zero hop) to full pressure. Fire a few
shots to set the lever to its optimum setting. Start at zero hop,
shoot a few rounds, adjust for more hop and observer the trajectory.
If the BBs tend to drop, increase hop-up. When they begin to magically
fly upwards, you're putting too much backspin -- lower the setting.
Additionally, excessive pressure and backspin may also result
in premature drop.
Warning
Since the hop-up mechanism imparts a spin on the ball, the spin
is relative to the orientation of the gun. If the gun is fired
sideways, the backspin is now turned into a sidespin (left
or right, depending on how you tilt the gun), causing a hooked
shot.