What engages the blade when you pull the engagement lever on a riding lawnmower?

Most lawn tractors don't have any fancy clutch system, but simply use the belt to engage and disengage the blades. When the blades are disengaged, the drive belt is slack and there isn't enough friction to the pullel to drive the blades. When engaged, the belt is taught and the belt drives the blades. Tension is controlled by a spring loaded pulley. When you pull the lever to disengage the blades, you pull this pulley away from the belt. When you engage the blade, the spring pushed the pulley into the belt, tensioning it.