To do this you need a gun in your hand, and on hotbar button (here I use UMP, and UMP is on 1 hotkey)
start jogging foward using W
Hold down Left mouse button (the fire button) and hold right click in bursts (like in video)
Keep tapping/pressing hotkey button 1 while doing above ^
Do it until you get the gun on your back, and you have to fists clenched behind your back (like Naruto)
Don't do keep pressing 1 after this happens because glitch will then dissapear

In this state, you can shoot yourself with gun on back, and have weird fists behind back when running.