A self-braking pulley is disclosed for controlling a rate of extraction of a tether wound upon the pulley. The pulley includes a spool for receiving a tether, the spool being arranged to rotate about an axle of the pulley as the tether is removed therefrom. The pulley further includes a braking assembly for applying a braking force to the spool and a drive assembly which is engagable with the spool via an engagement member. The engagement member is arranged to rotationally couple the spool with the drive assembly for driving the braking assembly, in dependence of a rate of rotation of the spool about the axle.