Duck and crawl are unique states since they seam to break the 'ground plane' rule. Due to the way the template script handles ducking, you should move your imaginary ground plane up by 16 units. This is because the script moves the player down by 16 units to allow them to duck and crawl under objects that they wouldn't be able to otherwise.
Standing and Ducking
This looks a little odd in MED but the effect looks correct in the game.
The 'duck' cycle is a transition state; it is called when the model starts to duck. At the start of this state the model is at its normal height, at the end it has moved down 16 units to its duck height. The duck animation allows the model to draw its legs up into its body. For the best effect have the feet of your model contact an imaginary ground plane that starts in the 'standard' (standing) position on the first frame and moves up to the 'ducking' (16 units up) position by the last frame.
The 'crawl' cycle is like the walk state. The model's feet should never penetrate the new ducking ground plane.