It is the Chevrolet drive belt tensioner and pulley that has replaced the need for periodic tightening of the engine's belt in between its replacement. This belt tensioner and pulley has a built-in spring which is what holds the tension against the serpentine belt that is mounted on your car's engine. This is why all of your engine accessories are now securely mounted in place with no need to adjust or reposition them to accommodate the old style tightening of the drive belt. The Chevrolet drive belt tensioner and pulley has another function other than just applying the required force to tightening of the belt. In the center of the pulley there is a bolt head to which a wrench can be applied to relieve the tension off of the serpentine belt. This is how the belt can be not only changed but change very quickly on your Chevrolet. The engineering behind the drive belt tensioner and pulley on your Chevrolet is simple. With the belt turning, the tension can be evenly applied to all pulleys from just one point of contact on the belt. Because this is a spring action of the drive belt tensioner and pulley, no longer will the belt be so tightly spun around a pulley that it wears on its bearings. This was a problem in the past for the alternator. The best way to inspect the drive belt tensioner and pulley is to have your Chevrolet running. By turning on the AC system, your AC compressor will cycle on and off. If this occurs and the belt does not slip, the correct amount of tension is being applied. If the belt slips on the pulley of the AC compressor when it cycles on, then not enough tension is being applied and an adjustment or replacement of this part is warranted.