The blade is fixed to the mast at a slight upward angle. At low wind speeds, the mast tilts right over, and effectively the blade stays out of the water with the assistance of that dangling buoy. But as wind speeds pick up and the blade starts spinning fast, it develops lift, much like a helicopter's main rotor, and begins pulling the mast upright.
Thus, in high wind speeds, it sits nearly flat to the horizon, greatly limiting the wind's ability to spin it faster. And as this happens, the buoy is lifted out of the water, becoming a ballast weight acting against the lift of the main blade, helping to reduce stress on the sea floor anchors and prevent the whole thing from taking off and starting a new life where nobody knows its name.
If it works it's brilliant.