Bucking?

I know how to slip the bucking onto the barrel and line it up, then slip it back into the hop unit. This, I think is the basic idea? But what I need help understanding is the nub, and where it goes, I have some madbull H- nub, or the fishbone on the way, and I need to know how to install it.
I believe that there is a groove in the hop unit that it sits in while you slip in the bucking and inner barrel, but i am not sure.

http://www.airsoftpost.com/bull-degree-shark-hopup-bucking-fishbone-spacer-p-28917.html

What type of gun is this for?

it is a g&p full metal hop up in a jg m4 cqb.

Then on top oh the Hop-Up unit when you turn the wheel you should see a little thing move up in down. Its called the arm. (As being pointed to in picture)


You need to take that out and it goes in there.

thanks for the help. And the picture helps greatly.
by the way, did you just photoshop that or find it like that. I don't have a good photoshop, or editing program on my apple.

For that one I just used paint real quick. I have photoshop though I only use it for creating like signatures and other cool graphics.