Place steaks on a clean surface, with a fork, poke holes in the steak. Place the steak in a shallow glass baking dish along with the soy sauce. In another small bowl combine mustard, garlic, brown sugar, salt and pepper, refrigerate mixture for 15 minutes. When 15 minutes is up take the steak out of the soy sauce, throw soy sauce away, rub the mixture over the steak. Refrigerate for at least 45 minutes before grilling. You can baste the steak with soy sauce while grilling if you like. Grill to your liking.