71 further has three hoofs 712 each having a pivot shaft 713 the hoofs 712 are respectively pivotally mounted on the base 711 via the pivot shafts 713 and positioned around the shaft 50 at equal angular intervals that is the hoofs 712 are arranged in a pattern centered at the axis of the shaft 50 with the pivot shafts 713 positioned at 120 degree intervals the axes of the pivot shafts 713 are parallel to the axis of the shaft 50 accordingly the hoofs 71 can be pivotally rotated between a braking position and a releasing position a resilient member 714 such as an extension spring is bridged between each two adjacent hoofs 712 for resiliently restoring the hoofs 712 from the braking position to the releasing position each hoof 712 has a substantially arc shaped hoof plate 7121 pivotally disposed on the pivot shaft 713 and a lining 7122 attached to a face of the hoof plate 7121 that faces the inner circumference 721 of the drum 72 accordingly when the hoofs 712 are positioned in the braking position the linings 7122 tightly attach to and abut against the inner circumference 721 of the drum 72 to apply a dynamic frictional force to the drum 72 under such circumstance the shaft 50 is restrained against rotation when the hoof section 71 is synchronously rotated with the shaft 50 under wind power a centrifugal force is created in direct proportion to the rotational speed of the shaft 50 when the rotational speed of the shaft 50 is approximate to a nominal upper limit t 1 the centrifugal force will overcome the pulling force of the resilient members 714 to make the hoofs.