Cut the pork shoulder into a few large chunks. You don’t want them bite-sized; I make mine about 3-4 inches.
Mix cumin, garlic powder, salt, coriander, black pepper, and cayenne together in a small bowl. Place the pork chunks in a plastic container with a lid or a Ziploc bag. Pour in the spice blend, then toss the bag vigorously until the chunks are completely coated on all sides.
Place the pork in a large, deep pan. Pour the lime and lemon juice into the bottom, then add water to just cover the meat.
Turn heat to high and bring the water to a rip-roaring boil. You want big bubbles! When it’s rolling, turn the heat to a simmer. Keep the pan uncovered. You want it bubble a fair amount, but not be a roiling boil. While it’s cooking, it will look like uninspired soup. Do not be discouraged! Essentially, you’re waiting for the water to evaporate from the pan, but while the water is evaporating, the powerful acidic qualities of the lime and lemon juice are tenderizing the meat. Yay, science!
At about the 2-hour mark, check the pot. The water should be much lower and maybe even almost gone. Now things get interesting… allow all the water to cook out of the pan and watch as the meat magically fries and carmelizes. It is a thing of beauty. But seriously, you need to watch it at this point.
Carefully turn the hunks of meat – without shredding them – to brown all sides, then remove the hunks to a plate and let them rest for 5 minutes before eating. If you’re into this sort of thing, you can save the pork fat from the bottom of the pan in a glass jar and use it for other cooking projects.