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.