Lay out the skirt steak in a dish for marinating (oblong baking dishes work great). (After seasoning, you can fold the steak, overlapping it onto itself, if it’s too long to fit in the dish.)
Sprinkle the cumin and garlic over all sides of the steak. Pour the lime juice, jalapeno liquid, and olive oil over the steak, and slosh the steak around to coat all sides. Place the onion and jalapenos under the steak, on top of it, and between any ends that were folded over. Cover and refrigerate. Marinate for at least 4 hours, preferably overnight. Turn the steak around in the dish once during marinating.
About 30 minutes before eating, prepare the Pico de Gallo and refrigerate.
Prepare a grill for direct cooking. For a charcoal grill, when the coals are ashed over, rake or spread them out in the bottom of the grill so the food can cook directly over the coals. (For a gas grill, fire up the burners so the food can cook directly over the heat.)
Just before grilling, pour the soy sauce over the meat, to help it brown. Grill over direct high heat. As the meat cooks, sprinkle on the liquid smoke, first on one side, then on the other. When the steak has good grill marks on one side, flip it over and cook until the interior is done, 6 to 8 minutes total cooking time n a really hot grill, or up to 10 minutes on a less efficient grill. Fajitas are best when cooked medium rare, slightly pink, and really juicy on the inside.
Let the steak rest 5 minutes before slicing. To serve, slice the steak into thin strips. Load a warmed flour tortilla with the steak, and spoon some Pico de Gallo on top. Tex-Mex heaven! (While the meat rests, you can boil the remaining marinade and onions at least 1 minute. Pour over the sliced steak before serving.)