Hi David!
It is not really such a good idea to step on an opponent's foot. There are three reasons for this. One, you have completely eliminated the use of both of your legs while you are stepping on their foot. Since one of your feet is not stable (the foot that is stepping on theirs), you cannot kick with the other. However, the opponent can still fire damaging close range kicks with the free leg, while you literally stablize their position by standing on their other foot! Two, if they suddenly jerk their foot out from under you, you are off balance and left in a bad position! Your structure is destroyed because of poor footing! Three, it is easy to twist or turn your ankle due to the positioning of their foot at the time you step on it! I do advocate close range foot stomps, but not stepping on the foot with the intention of remaining there for more than a split second!
Leg checks/traps work whether the opponent is in a right or left lead. You must, however, remember to go shin to shin with your contact when in matching leads. When in mis-matched leads, always step to the outside (behing the ankle/against the outside rear of the calf) of their lead leg. Stepping to the inside offsets your stance alingnment and leaves you in a vulnerable position!
Hope that helps!

Keep Blasting!
Sifu Lamar M. Davis II
Senior Instructor
Hardcore Jeet Kune Do
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