Remove the silver skin from the back ribs and cut into individual bone segments. Place them in a large mixing bowl. (If using a barbecue grill, do not cut into segments. Leave the racks whole.)
In a blender or food processor, combine shallots, lemongrass, garlic, fish sauce, dark soy sauce, sugar, water and thai chili. Blend well. Reserve ½ cup of marinade and set aside. Pour remaining marinade evenly over the ribs and massage it into the ribs. Cover and let marinate for 1 hour or overnight.
Place a trivet in the inner cooking pot of an electric pressure cooker and add 1 cup water. Add the ribs.
Close and lock the lid, making sure the steam knob is on Sealing/Locked. Pressure cook on high for 18 minutes. If you prefer the ribs less fall-off-the-bone and with a little tug on them, cook them for only 15 minutes. If you want them to completely fall-off-the-bone, cook for 20 minutes.
Once the cooking is done, quick release the pressure.
Preheat the broiler. Line a sheet pan with aluminum foil. Lay the ribs down and brush with the reserved marinade. Broil for 3-5 minutes until the ribs are browned and caramelized.
Flip the ribs over and brush with any remaining reserved marinade. Broil for 3-5 minutes until ribs are browned and caramelized.
Alternatively, grill the ribs on the barbecue and baste with the marinade until browned and caramelized.
Garnish with cilantro and chilis.