The purpose of these depletion workouts is to truly deplete muscle glycogen. The type you are referring to are usually the "metabolic" type workouts, where you want to do high rep exericses moving quickly for maximum fat loss. Calorie and fat burning isn't the objective here (although some will happen, of course, yay); diet takes care of that part.

Using isolation exercises as much as possible will allow you to target the muscle you're trying to deplete, as you will only deplete glycogen from the muscle that's being worked. What I found out yesterday is that not having access to machines sucks in this case! My poor arms are getting hammered from having to hold dumbbells and barbells.

The point of UD2 is to fully deplete muscle glycogen during the fat loss phase, setting yourself up for a supercompensation during the carbup, and therefore allowing for a growth phase on the weekend. So hopefully you can have the best of both worlds--fat loss on the diet days, and muscle growth on the higher cal/carb days. At the very least, you'll maintain muscle, which can be hard to do when you get very lean.

I'm not what I would consider "very lean" yet, but right at the upper edge, so figured time to give it a go.