It sits on the part hat moves up and down when you adjust the hop up. Don't know your gun, but flip the hop up unit upside down and look for a recatungular hole. Need to put that rubber tube sidewyas in that hole, after make sure it goes up and down when you adjust the hop up. Then put oil on your rubber sleeve, twist and press the barrel with rubber into the hop up unit. Once in all the way, point the barrel towards a light area, adjust the hop up setting up and down and make sure it goes up and down as it should.
Nutshell, lose that little peice, lose 100% of your hop up adjustment.