In my school, defensive stances are really important. Defensive creates the offensive. That is what my coach always said. In defensive stances, you always want to be lower than your opponent because you can be quick and fast. Secondly, you want to have a straight back when defending because your back will start aching. Lastly, when defending you want force your opponent to base line or to a side where there are help. If you are defending on the top, force the player to the side where he is weakest. For example if he is a righty, force him to the left and vice a versa.