Basically, with DUP, you are changing your rep schemes each workout for muscle X in your workout schedule. So for example, if you have a 7 day schedule where you train triceps in two different workout days, the first workout may be done with sets of maybe 3-5 reps, while in the second workout, you may do 10-15 reps.
By doing so, what I have found, is that you can use both "strength building" rep schemes and also "hypertrophy" rep schemes within the same schedule, potentially giving you the benefits of both. I can only speak for myself, but when I did this workout last summer, I was on the most intense cut I had ever been on, but I still made huge strength increases, some pretty darn good mass increases, and was definetly at a peak in how my body looked. I was ripped! I kinda wish that I would have done a better job at keeping that shape, but I wanted to bulk but at the same time got a full time job that made me work out only 1-2 times per week and at the same time hold a diet that was less than optimal for a clean bulk.