'''What’s a Derailleur?''' [ … ] The derailleur is your bike’s gear shifting device. When you adjust the shifter on your handlebars, the derailleur pushes your bike chain to the next gear over, and then adjusts the tension onlength of the chain to accommodate the change in gear size. [ … ] The derailleur is controlled by the shifter through the cable that connects the two. When the cable is loosened, the derailleur moves downward towards the smaller gears.