A sensor detects the lack of blade rotation which then causes a mechanism to operate which switches the batteries to a dc motor which drives the blades.
And as a back-up - if the demand on the turbine has been too much during the day to charge its own batteries, you could use the off-peak power supplied to the grid by my hybrid car to charge them up.
Blimey, all this endless debate by the so-called 'experts', yet we've come up with an integrated energy policy in a few days. Just goes to show, doesn't it.