'''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 on 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 outwarddownward towards the smaller gears. As it does this it also tightens the bike chain, to pick up the slack created from going from a larger to smaller gear. Conversely, when the cable is tightened, the derailleur moves inward towards the larger gears, and loosens the bike chain, giving slack to compensate for the increased tension that is created from going from a smaller to a larger gear.