The winding is achieved by an electric 12 volt motor at the rear of the movement. As the clock unwinds the weights drop lower and a trip arm attached to one of the weight's pulleys gradually pushes against a cam attached to a mechanical switch. This can be seen at the front of the movement as shown below.
Eventually when it is low enough the cam triggers a switch and the motor starts rewinding. As the weights then travel back upwards they will trip this mechanical switch again and switch the motor off when they get to the top.