Often what happens is the driver will over correct and continue to over correct until losing control. The best way to stop fish tailing is to apply just the trailer brakes manually and steer straight. The weight of the trailer and braking will straighten everything out and then you can continue forward.
I once woke up while my DW was driving, the truck and trailer were swinging back and forth wildly, she kept sawing back and forth on the steering wheel. I grabbed the steering wheel held it straight and told her to apply the trailer brakes. All was well and I tried to go back to sleep.
