Pat ribs dry. Apply a generous amount of rub to both sides of the ribs (4 to 5 tbsp). Transfer to an aluminum pan and refrigerate 30 min to 4 hr until rub looks moist. Stir mustard and 1 tbsp rub into Mop Sauce; set aside.
Heat grill to medium low (250°F) for indirect cooking and arrange smoker box or wood chips in grill. Place ribs in the pan on grill away from heat. Cover grill and cook, basting gently with Mop Sauce after 30 min and every 20 to 30 min until ribs are tender, 3 ½ to 4 hr.
Ten minutes before removing ribs from the grill, baste with barbecue sauce. Remove ribs from grill and let rest 10 min, then cut and serve with more sauce.
The secret to KC-style ribs is mopping and wood chips. The mop sauce soaks in as the ribs cook, making them melt-in-your-mouth good! Wood chips add that authentic smoky flavor.