The science behind this is easy man, your muscles have memory, they remember everything you do, if you keep doing the same thing over and over then your muscles get to the point that they "guess" whats coming, at this point the growth stops, by including a "light day" on each muscle group or a day of drop sets or anything else you keep them guessing, on "light day" it doesn't necessarily mean the whole day has to be light, I tend to take one exercise at 30% max and do one set of that exercise to failure (I shoot for 40 reps). then move onto the next exercise, giving a longer recovery period between exercises (about three to four minutes).