The Front Kick is a basic but effective Karate technique (the basics are the most important). The knee is chamber and the ball of the foot is extended into the target and then the knee is recoiled. The kick can be snapped or the hip can be thrusted forward. The snap creates force from the snap and the thrust is used to drive through and knock an opponent away. Look at the clips below.
Anderson Silva's Front Snap Kick
Lyoto Machida's Jump Front Kick