I wouldn't do any kind of intense cardio. Some mild cardio would be best. HIIT would pretty much be the worst thing you can do if your goal is to decondition/deload your legs.
I would adjust calorie intake on those days to compensate, so that you are still at maintenance, or if below maintenance then only very slightly below. Quad is right, you won't lose tons of muscle mass during a two week SD unless you starve yourself the whole time, but you still want to be more safe than sorry. So keep the cals around maintenance and don't go overboard on cardio.
From the FAQ:
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CARDIO DURING SD
Will serious cardio sessions during the SD week hamper your results?
Well, it will have the greatest conditioning effect on those muscle groups used for the cardio.
Exercising has the opposite effect as Deconditioning.
By doing intense exercise you are conditioning the muscle to be resistant for future exercise bouts. Chronic aerobic exercise interferes with strength and size gains on a number of different levels (CNS, MHC expression, etc), none of which have anything to do with deconditioning.
I haven't found brisk walking (on a good incline) to significantly hamper deconditioning.
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