Kicking open a locked door should only be attempted during emergency situations. If you are trying to escape a fire, make sure you check the door, doorknob and crack for heat with the back of your hand before kicking it. If it's hot, avoid opening the door. (See Escaping a Fire) The following kicking technique could save your life.
The front snap kick is directed forward toward the opponent's stomach, sternum or crotch, and with practice toward the face. Without shoes the striking surface is the ball of the foot. However, with shoes on, which is the more likely state you'll be in when using self-defense, you should kick with the toe of the shoe if it has enough support. You begin the kick by lifting the knee directly in front of you. Then extend the foot to strike. To speed up this kick, concentrate on lifting your knee as fast as you can. This will naturally increase the speed at which your foot extends toward the opponent.
This kick is used usually for someone behind you. You simply lift the leg farthest from the target (your front leg), and kick back with the heal of the foot. This kick is extremely powerful. However it is more difficult to aim and should be practiced with a heavy bag or other target.