The hopup? Well...let me try and explain this:
The inner barrel of an airsoft gun is attached to a Hop-Up Unit. The way this works is the top if the inner barrel has a small, square hole cut into it (close to where the BBs enter the barrel from the feeding tube, which is generally integrated into the hop-up unit itself). The end of the barrel which is inserted into the Hop-Up Unit (including that square hole) is covered by a rubber sleeve called a Hop-Up Bucking. Inside the Hop-Up Unit, there is a small piece of plastic or rubber called a Hop-Up Nub. The Hop-Up Nub sits on top of the Hop-Up Bucking over that square hole, and when the Hop-Up Unit is turned "Up", the Hop-Up Nub pushes the Hop-Up Bucking down through the square hole until the Hop-Up Bucking is actually in contact with the BB itself. The purpose of all this? When the BB hits the protruding portion of the Hop-Up Bucking, it puts a backspin on the BB which in turn makes the BB fly farther.
Did I miss anything?