Ji is a "squeeze" or "press" (depending on the translator). This is fairly easily manifested in thelegs, and is one of the simplest energies to execute.
The two examples I cited in the original (in "Push Legs" under the "Push Hands" discussion) exemplify Ji as it applies to the footwork. Locking the ankle and twisting to apply force to the knee is a clear manifestation. In fact (I agree), any step where the foot turns out can serve this function. Turn the foot behind the opponent's ankle and shift the weight IN THE SAME DIRECTION AS THE FOOT as you go to step through -- this is important to avoid straining your own knee in the process. As you shift weight, your knee presses against the opponent's knee. To stay on line, it is best (for this application) to pull your weight forward with the lead leg, rather than pushing with the back leg. This changes your lead to substantial earlier and helps keep the balance while moving.
Another example is used in close combat. With your weight on your back leg, make contact between the side (inside or outside) of your foot and your opponent's foot. Shifting forward will apply Ji to the leg or knee, though the ankle will not be locked.
Master Eddie Wu (Kwong-Yu) -- 6th generation Grandmaster of Wu Family Taijiquan -- showed us an interesting version of Ji hidden in the stepping. In forward stepping, the heel touches first, then the rest of the foot sets down as the weight shifts forward. To make this Ji, the heel is set on top of the opponent's instep. As the weight shifts forward, the rest of the foot presses against the ankle, effectively pressing your entire weight against the joint and either forcing the opponent back or keeping him from moving forward. This works best in short range stances (because of balance), and only if you have developed a strong root.
Once again, these methods can cause permanent injury, so BE CAREFUL.