The upshot of the Texas Crutch technique is: after about 2 hours on the smoke, the internal temperature of a brisket will plateau for 6-8 hours before climbing toward its final temperature. According to experiments by Greg Blonder[0], this stall is caused by evaporative cooling: once the internal temperature reaches a point where the energy lost to evaporation balances the energy gained from the (low---225-250 Fahrenheit is the "right" temperature for a brisket) ambient temperature, the internal temp of the meat will stay put until sufficient water has evaporated.
Some call this cheating, but in my experimentation the "crutch" produces significantly tastier results because of the difference in juiciness. As additional evidence here, I'll note that every BBQ place in Austin whose brisket is good to great---Franklin, La Barbecue, John Mueller Meat Co., and even Rudy's---wraps their briskets in foil. (Edited to add: I didn't mention Black's and Kreutz's because I'm not 100% sure that they do, but I vaguely recall that they do.)
One worry might be that the meat doesn't absorb as much smoke after the foil is applied, and this is indeed true. One can trade off additional time in the smoke for more evaporation. After about 4 hours, the meat isn't going to absorb much more flavor from the smoke, so that's a great time to wrap it.
In fact, at that point you may as well make your life easier by transferring the meat to an oven (ideally, a convection oven to avoid hot spots) and finishing it off. It may feel wrong, but in my experience the results are superb.