Lets say we want the trailer to turn in a particular direction. If you had hold of the tow hook and where walking with the trailer pushing it backwards you would

push the back of the trailer in the opposite direction so that the front would going the direction you desired. If you now think of the back of the towing car you need to make the back of your car push the back of the trailer in the same. You would turn the wheels of the car to the opposite direction that you wished the trailer to take. When you reversed this would make the back of the car move away from the direction you wished to reverse to but that in turn would turn the trailer in the desired direction.

However as you continue to reverse the car would continue to increase its direction away from the corner and therefore your trailer would continue in the opposite direction until the side of your car would touch the side of your trailer. This is called jackknifing and needs to be avoided. We need to take off the lock to reduce this and then follow the trailer in reverse.