As said above, wrapping accelerates the cook since it causes the meat to steam or braise in juices in the foil. Another thing it does is keep the ribs from getting really dark since wrapping keeps the smoke off the ribs.
I agree that the 321 only works at a lower temp like 225 or even a little lower.
As with most all things BBQ, there's no real absolutes with this technique.
In competitions, we cook at 300 degrees and we do more of a 2-2-0.25 thing.
Normally, I'd recommend just cooking things until they're done, whether wrapped or not. With the wrap, however, it becomes a little harder to check the ribs without UNwrapping them, but when you do unwrap them and open the smoker, you've just increased the overall cook time, so I think that's where this "magical" formula came into play.
What I've found that works well is 3 hours in the smoke, 1.5 hours in the wrap and then after unwrapping, cook until done, which usually ends up being 0.5-1.0 hour (this all done at 225 degrees). The shorter time in the wrap will prevent you opening them up and the ribs being mush. This way, you can judge how much longer they need after you unwrap.
While it is a proven technique, it's not really any more complicated than wrapping the meat with foil or paper, just like with brisket, pork, chuck roast or any big meat. Same technique, it's just that somewhere along the way, someone figured out what cook times in and out of the wrap worked at what temperature and it became a "method".
Try it out. You may like the results. Or you may not. You can then adjust to what your preferences are and BOOM.....you'll have a new method! :becky:
