In windsurfing, the failure of function of the fin equivalent to a stall of an aeroplane's wing.
When planing, the fin works like a wing, cutting through water at a small (several degrees) angle of attack, and countering the lateral drift caused by the wind. The sailor's foot pressure leverages the lift, keeping the board flat on the water.
=======wind=====>>
\ Force from the feet
V____________
/ board \
~~~~~~~~+------------+~~~~~~~
- ||f -
Lift of <------||i -
the fin ||n -
- - \/
(rear view)
If the pressure is too high, the fin loses lateral resistance as the angle of attack jumps up to around 45°. The back of the board is suddenly thrown leeward. Usually a bubble of air forms behind the fin.
Experienced windsurfers are able to recover from spinouts by guiding the board to the correct position with their feet, thus restoring the angle of attack of the fin. Furthermore, experienced windsurfers sense the moment of spinout and sheet in just as much as possible to bring the gear to the verge of spinout without actually tripping it. This is the fastest mode of the windsurfing board.
Excessive spinouts can be fixed by putting on a larger fin (or a smaller sail). Too big a fin though will deliver too much torque to the sailor's feet, and thus will be quite tiring.