Motion, The ball of foot is applied to the target by either a snapping or thrusting motion. For the snap kick, the knee is pointed at the target and the lower leg is snapped into the kick with a thrusting motion. The knee must be raised high enough that the foot does not just slide up the surface of the target. For the thrust kick, the knee rises higher than the target and then drops as the foot extends and the hip is thrust behind the kick. Although the leg and knee may move in many directions, the foot itself moves in a straight line from the floor to the target.
Direction. Most people think of a front kick as being executed toward the front, but, unless you are righting from a basic front stance, you will actually be executing the front kick toward the side. For example, think of firing a lead leg front kick from a back stance. The kick is actually firing out to the side.
Contact. The traditional front kick uses the ball of the foot to strike the opponent. This is an effective way to kick, since the striking area is small. It focuses the power, and the padding on the sole of the foot protects the bones of the foot from injury. To kick this way, the foot is angled forward and the toes are angled backward to protect them. When the toes are angled backward, they will naturally flex backward even more as impact is made. Pointing the foot and pulling the toes back by muscular action slows the kick because it stiffens the movement of the knee joint. A compromise is to pull the toes back only enough to prevent them from striking first and then letting them flex as needed. The instep of the kicking foot should line up with the shin. Dropping the heel too low causes ankle to flex and it may collapse, causing injury during a hard impact. Practice correct foot position by standing on the balls of the feet with the heels held high. Some styles, such as Uechi-ryu, actually use the big toe as the striking area for the front kick, but this is dangerous to the kicker. The heel may also be used as the striking area. The toes point upward or outward and the heel is pushed forward.
Variations. Front kick may be performed with the leading or trailing foot. Since there is little hip twist and less acceleration distance, the leading leg kick is faster but less powerful than the trailing leg version. However, it is useful for checking an onrushing opponent since the opponent supplies much of the impact force. With the lead leg kick, be prepared to project weight forward, otherwise recoil may drive you backwards. Jumps may be added to front kick.
I once had a Korean instructor who would fire a front kick toward your face, let his foot drop down to your chest, use his toes to grab the front of your uniform, and then pull you in range for a series of hand attacks. He could use his toes as easily as you used your fingers.