Slice the beef skirt and transfer to a baking dish or pan. Set aside.
If you have time, you can lightly toast the peppercorns and cumin seeds to bring out more flavor. It's optional.
In a molcajete, blender or coffee grinder, combine the salt, peppercorns, cumin seeds and oregano. Blend until you have a fine powder. Tear the chile ancho into pieces and add it to the blender along with 1/2 cup of oil. Blend until smooth.
Pour the blended wet seasoning(marinade) over the meat. Use you hands to really work it in so it's evenly coated. Set aside.
To the bottom of the pressure cooker, pour in 3 cups of water. Arrange 2 sections of the banana leaves so they criss cross each other, making sure the flaps reach top of pressure cooker.
Layer the seasoned meat evenly in pressure cooker. Add the onions, celery and bay leaves. Pour in any remaining blended seasoning with oil over the top. Cover everything with 2 remaining banana leaves. Fold in the flaps and push down slightly.
Secure and lock pressure cooker lid onto the pot. Heat to high until the safety valve seals shut and pressure cooker is secure. Once it begins releasing that steam rapidly, reduce the heat to just below medium and set your timer for 1 hour.
While the arrachera is cooking, prep any salsas, garnishes or sides you may want to serve.
After one hour, shut the heat off from the pressure cooker. Now you wait. Wait until the safety valve opens back up or you can carefully release the steam. Be careful.
When ready, transfer the barbacoa to a serving dish with a lid. Serve with warm tortillas, guacamole, salsa, limes, cilantro and onion. Buen Provecho! Y