When triggered, the characters suddenly find themselves moving in the opposite direction, back the way they came. This will occur even if the characters are not touching the floor (Levitation spell, magical flight, etc.). The characters can turn back and try again, but the same thing will keep happening.
The trap cannot be disarmed or dispelled, and can only be defeated by proceeding forward while walking backwards. By doing this, when the characters are turned around by the trap, they are facing the right way again and can just proceed forward once more.
Optional: Even teleporting past the trap won't help, as the character(s) will just be thrown back to a point close to the beginning of the affected area, facing the opposite way.