Jamming is stopping an attack either by getting so close to the attacker that he or she does not have enough space to fire the attack (or not enough room to generate full power in the attack) or by using a body part to stop forward movement of the attacking limb. It takes less effort to prevent an attack from moving than it takes to stop the attack once it is moving. In addition, jamming frustrates the opponent because he or she never gets to fire the attack.
Foot jamming is when you place your foot on the floor in a position that prevents the opponent's foot or leg from moving. Even under tournament rules that do not permit kicks to an opponent's legs, these movements may be made subtly so they do not appear purposeful and thus will not draw a warning from the referee.
If you are within hand attack range, you are probably within range to jam a foot. When you are in close range, slugging it out with an opponent, punches and blocks flying, there is little time to think, however, a well-placed foot can distract or unbalance the opponent, creating an opportunity for you to apply the finishing blow.
If you are in a low stance, your lead foot will be close to the opponents lead foot, or you could purposefully let the lead foot creep forward until it is close to the opponents lead foot, or you could just step your foot into position. Either way, once you foot is in position, it may be used to prevent or impede movement of the opponent's foot.
Stepping on top of your opponent's foot will jam it. This happens many times in sparring matches by mistake. It usually results the opponent either stumbling or falling. Even if this does not happen, the opponent will be locked into position and vulnerable to attacks. If stepping on the opponent's foot is done carefully and not very often, it will not draw suspicion from the referee.
If you slide your foot behind the opponents' foot and lean your shin forward into the opponent's shin, the opponent's lower leg will be locked into position, making the opponent vulnerable to attacks.
If your leg is outside the opponent's lead leg, you may push you knee against the inside of the opponent's lead leg to make it slightly buckle, cause instability, and create openings. If your leg is outside the opponent's lead leg, you may push against the outside to the opponent's lead leg to make it slightly buckle, cause instability, and create openings.
Even when foot jams do not work, they may unnerve the opponent or at least give him or her something else to think about.
Kick jamming is stopping a chambered kick, such as a round kick, from firing by using your hand, foot, or body to prevent it from moving toward you. For example, as the opponent chambers for a round kick, use a heel kick against the shin of the opponent's kicking leg to jam the kick.
Hand jamming is stopping a chambered hand attack, such as a jab, from firing by using your hand to prevent it from moving toward you. For example, as you initiate an attack, place you hand in front of the opponent's lead hand so a jab cannot be fired.
Body jamming is quickly closing the range when the opponent attacks so your body may stop the attack from firing, or so the attack does not have enough distance to generate full power. When fighting a kicker, continuously staying close to the opponent prevents him or her from kicking.