Whisk together the brown sugar, smoked paprika, onion powder, garlic powder, and salt. Set aside.
Remove membrane from ribs. Place ribs, bone side up, on a large cutting board. Slide the point of a paring knife between the thin, white membrane and the rib. Lift.
Once enough of the membrane has come away from the rib, remove the knife. Grasp the membrane with your hand and pull away from the rib. Usually, it comes off in one large piece. If it tears, repeat, as needed to remove all the membrane.
Turn ribs over. Rub ¼ of the prepared rib rub into the ribs. Turn over. Repeat. Do this for each rack of ribs.
Add ½ cup BBQ sauce into the insert (cooking pot) of an electric pressure cooker. Whisk in a half cup of water.
Whisk together the ketchup, water, molasses, and cider vinegar until smooth.