The Belt Connects To The Trolley And Drives The Trolley Along The Rail To Raise And Lower The Door. When The Belt Breaks Or Is Worn Replace It With A 7' Or 8' Belt Of The Same Type For Your Opener. Older Belts Come Without A Inner Slide For The Trolley And You Must Reuse It If The Old Inner Slide Is Good. When The Inner Slide Is Bad Do Not Order The Belt Alone There Are No Replacement Inner Slides For The Older Belts.