Here's an old thread from good ol'Lyle MacDonald that should at least give some kind of scientific explanation for this thing called SD.
<div></div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">Two goals of SD:
The first is general recovery (neural, muscular) after the entire cycle.
The second goal is as important: detraining lets the increased connective tissue go away so that you can get more easy damage when you start the next cycle.
That is, at the end of 8 weeks of continuous training (or whatever the exact numbers are), damage is too hard to generate because of increased connective tissue (and the practical realities that you can only add weight for SO long before you get hurt).
A week off (strategic deconditioning) lets some of that CT adaptation dissipate so that damage (and hence growth) is easier the next cycle.
You also have to hope that connective tissue adaptations go away faster (or to a greater degree) than the size increases, which is supported by data (the early de-adaptation to training is neural first).
As well, if you did the eccentrics, you should have a delayed training effect that keeps growth going as the connective tissue is going away. Well, hopefully.
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and...
<div></div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">First and foremost, the week off will give you some time to heal (on top of another critical component).
As well, starting with the higher reps gives you more time to heal, jacks up blood flow to the joints, and has hormonal effects that may help with connective tissue healing and injury.</div>
and...
<div></div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">Ok, negatives and the week off since they are related. If there' a reason folks stop growing (well, there are lots of reasons) here's the one nobody every mentions.
As you train, your body increases the amount of connective tissue within your muscular structures. This is geared towards one thing: decreasing muscle damage from training. </div>
Yeah, I think that about sums it up.
So as Colby pointed out, during this rather unwanted period
keep up the protein intake and nutrition unless you have put on more fat than you desire (in which case you cut down 500 calories or so) and your muscles will end up saying "thanks for coming"