Mix together salt, coriander, pepper and allspice in a small bowl. Rub flesh side of both racks of ribs to coat with spice rub.
Pour water, vinegar and Worcestershire sauce into a pressure cooker, and add onion, garlic and cloves. With the side that has the rib ends protruding facing down, gently coil rib racks to fit one inside the other in the pressure cooker. Secure lid, and heat to high pressure. Cook for 20 minutes, and then slow release.
While ribs are cooking, heat a grill for indirect heat. (Place a smoker tray with soaked and drained smoke chips directly over the heat, if you like your ribs extra smoky.) Pour 1 cup of barbecue sauce into a small bowl for basting ribs, and heat the remaining 1/2 cup in a small saucepan over low heat to serve on the side.
When pressure has eased from the pressure cooker and any flames have subsided from the charcoal, carefully transfer ribs flesh-side up to the indirect heat area of the grill. Brush liberally with 1 cup barbecue sauce, and grill covered, for 10 minutes. Turn over, and baste again two more times (ending meat-side up) at ten minute intervals, for a total cooking time of 30 minutes.
Serve remaining barbecue sauce on the side.