Hop up units and rubbers are a vital part in your airsoft gun (both gas blowback and AEG). The hop-up unit and rubber are responsible for putting backspin on the BB to make it spin in flight – This gives the BB lift thanks to the Magnus effect. Hop up units differ depending on the AEG model that it is designed to be installed in and some systems will even have bespoke hop units – Like VSR10’s and some other sniper systems.