The retreat is needed to maintain correct distance from your opponent or to get your opponent to come forward, creating opportunties for you to score.
Move your rear foot back first , then immediately move the front foot back.
Keep your feet within 1 inch (25 mm) of the floor. Retreat on the balls of your feet. Flex knees and press them forward, keeping your knees over your shoelaces.
You will feel like you are moving your front foot half the distance your rear foot moves and twice as fast.