Mad Bull's Ultimate Hop Up features 3 Functions in 1: Tracer unit: You can attach a MadBull LED tracer unit on this Hopup unit. This is an optional accessory and must be purchased separately. Chamber lock: With this new hop up design you will not have to worry about losing and dropping several bb's with a magazine change. This new design provides you with a way to keep bb's in your AEG and ready to use and not on the floor and even better it stabilizes the top of the magazine to ensure a secure and flawless feed of bbäó»s into the AEG. You can take out the oring if you don't like this function. This function is designed by Brian Holt. Easy adjust Hopup: Compared to traditional Hopup, our Hopup can be adjust easily.