Shorting the mill puts a big drag on the blades, slowing them. If the drag is adequate (i.e. the alternator is sufficiently "powerful", the coils and wiring are low enough resistance, and the wind isn't so high that the torque overcomes the drag and keeps the mill spinning fast) the blades stall, greatly reducing the torque, and the mill slows further.
The mill quickly ends up spinning very slowly - just fast enough to create enough generation to produce enough counter-torque to balance the startup torque of the stalled blades.
But there are two downsides to this:
1) When you short it you dump the energy from its inertia into the coils as heat. You also produce an extremely high current, which suddenly produces a correspondingly high torque, jolting the mill's structure. (But you don't want to put this load on slowly, because that leaves the wind pumping in more energy to be dissipated as heat in the genny during the load ramp-up.)
2) If the braking isn't adequate to stall the blades you end up dissipating a LOT of power continuously in the genny, likely destroying it.
You could start the braking with a load resistance in series with the short, to dissipate some of the power from decelerating the rotor and blades. But the more resistance, the less braking torque, the longer the mill spins, the more power it collects to be dissipated in the braking system, and the lower the wind speed where the wind overcomes the brakes and keeps the mill spinning (and burning up).
