Snap Shooting is a very hard skill to master, but essential to playing paintball. While behind a bunker, the player must lean out and release one or two quick shots and get back behind cover without getting hit (the number of shots depends on the marker, player's skill level, and distance to the target).
First, crouch with one knee on the ground. The grounded knee should be on the side you intend to lean. Know where your target is before you lean! Point the marker in the right direction, so that all you have to do is lean laterally out. When you lean, don't lead with the marker! This tells the target that you are getting ready to come out from behind your cover, and you can catch one in the face. Lean out, shoot, lean back in. Don't wait until you are fully exposed to shoot. Anticipate your moves. It's ok to actually shoot your own bunker (even if it does make you feel a little stupid). Don't wait to watch your shots, this gets you killed. Besides, the screams of pain will be a good indicator.
Learn to shoot left and right handed. A person who always leans out of the same side of the bunker is predictable, and an easy target.