The answer is moment enertia, same as keeping a cat on one hull.

By moving out the moment of enertia increasing slowing down the angular accellerations give a human mind enough time to react by steering and using the centrifugal forces of steering a curve to restore the balance. There is also a 3rd very important factor and that is the sing of the rig. It acts as a very large daping device. When the rig falls to luff the angle of attact is increased and so too the lift, result the angular accelleration is much reduced. All works the other way around when falling to lee. This factor also works when boat is moving very slowly.

I mean how do we human being rid a bicycle and even without any hands on the steering bar after we have gained experience enough ? Same principle. It gets harder with less speed as you can't make the S-curved to regulate the instable system as effectively as before.

This all is fun stuff, can you tell that this is my field. (control engineering)

p.s. in gusty conditions these guys use flexible masts that bend away in the gusts overwise the reaction time would be too small for a human being.

If you want some quick response on a sailcraft than try land- or ice yachting (with exception of the DN yacht with there VERY bending and yielding rig). These craft sail on two wheel or two scates nearly all the time with enough wind. And at those speeds the response can be quite rapid (not much enertia helping your there. You'll need a soft hand on the steering.

Wouter