Cut the pork in about 3 to 4 pieces so it's easier to manage.
In a mixing bowl, combine chili powder, paprika, cumin, garlic powder, salt, and pepper.
Evenly coat the pork with the spice rub; cover and place in the fridge for 30 minutes.
Remove from fridge; cut several slits in the meat and stick garlic cloves inside the slits.
Press “Sauté” on the Instant Pot and heat the olive oil.
Add the pork and brown on all sides; this may have to be done in 2 batches.
Add onions, 1 cup barbecue sauce, and water or chicken broth to the meat inside the instant pot; stir to combine.
Close the lid, turn the vent to “Sealing” and set the instant pot/pressure cooker to “Manual” for 60 minutes.
When time is up, let the vent naturally release. DON'T do the quick release option.
Open the lid and, using a slotted spoon, transfer the pork to a large cutting board or plate; set aside.
Press the “Sauté” button on the Instant Pot and let the sauce simmer for about 10 minutes, or until it has reduced by half.
In the meantime, using two forks, shred the pork.
Once the sauce has reduced, add the shredded pork back into the sauce; stir until well combined.
Serve immediately with extra BBQ sauce on the side.