Tom, they do apply brakes (electrical and mechanical) , but that's not the main means to stop them turning. They 'feather' the blades, (they are variable pitch turbines) so that the angle of attack to the wind is 0°, meaning no 'lift' and hence rotational force is generated. Also, they face the turbine assembly into the wind, no matter which way the wind is blowing. (OT: have you ever seen a turbine nacelle 'unwind' its cables? Interesting. Look it up) Things you might not have known.