I've tried german volume training twice in the past with moderate success.When I put my cousin on it, his size and strength levels blew up.But, his recovery ability has always been above average whereas mine sucks.And this type of training is very taxing.As for the science behind it, I'm no exercise physiologist but here is one of the references I've found when researching the topic.
Scientifically speaking, by exposing a group of motor units to such a high volume of work, the body adapts to the incredible amount of stress by growing the targeted fibers at a pretty fast rate.And, along the same lines, the large training volume appears to contribute not only to increased mass, but decreased body fat (Hather, et al, 1992; Stone, O'Bryant, and Garhammer, 1981)
As you can see from the dates, this isn't a new system and I've heard many lifters from Germany used this system as far back as the 1940's.
The nuts and bolts of the system are:
1) Pick one exercise per muscle group
2) Pick a weight for each exercise that is either 60% of 1rm or a weight you could do for 20 reps in good form.Now do 10 sets of 10 reps with between 30-90 sec rest intervals.
3) Each muscle group is worked only once per week.
4) Use this system for atleast 4 weeks but no longer than 8 weeks before switching programs are performing a variation of GVT.
Here is one of the programs I used:
Day 1(Chest & Back)
Bench presses
Bent-Over Barbell Rows
Day 3(Legs)
Back Squats
Lying Leg Curls
Day 5(Shoulders & Arms)
Standing Press
Standing Curl
I found it beneficial to do supersets on this porogram with 30 seconds rest between exercises.The only other exercises I did was a few supersets of crunches and back extensions on my leg day.