A blade for a wind turbine includes a device for causing air at an increased velocity and/or pressure to escape from exit openings in the blade, thereby to apply a force. Pressure and diameter adjustable valves may be provided at each exit opening or orifice to allow the air to exit from the blade only when the air pressure within the blade exceeds a preset level. Also disclosed is the provision of valves at the exit openings or orifices on an opposite edge of the blade that allows air to escape from the blade to slow down but not stop rotation of the blade when a dangerous wind condition is detected.