The Husqvarna 579150802 Lawn Mower Pulley Idler Backside maintains belt alignment and tension in the drive system of compatible lawn mowers. This flat idler pulley contacts the back side of the belt and rotates as the belt moves during operation. It works by guiding the belt along its path while applying controlled tension to prevent slipping and ensure smooth power transfer between pulleys. Replacing a worn or damaged idler pulley restores proper belt tracking and supports consistent mower operation.