The full concept behind the combination of water depletion & carb deplete / load - please keep this in context tho - I'm a BB competitor and its much more important to get the ripped veiny look than for figure - so the shorter water deplete may work sufficiently.
As nikkita said, carbs help to give you the filled out look and they also require water to hold them. So just like the first 2 weeks of the Atkins diet - you drop your complex carbs for say Sunday - Wed prior to the show. If you've ever carb cycled, you know how shitty you feel when you are completely carb depleted but havent' quite quite switched over to a full ketogenic state. But you also expel water like crazy. So that coupled w/ a planned water depletion reduces the total amount of water in your body. If you throw in some potassium sparing OTC diuretics like MHP Expel or Taraxatone (dandelion root + usa urvi + potassium + mag), you can further promote water dumpage.
So by Wednesday you pretty much feel exhausted, bitchy & like complete shit because of the carb deplete. Then you start the carb load on Thurs - Sat. Carbs require 36-48 hrs or so to start filling you out. Lots of people tend to look better (more filled out) on the day AFTER their show because they didnt' carb up soon enough. But again, for figure, its not as critical - you want the softer look but not so soft that you lose your 6 pack & show squishiness in your ass. So as you start the carb load & continue the water depletion, you are further reducing your water intake, but the carbs are going to start looking for water to help them fill out -- so they will further start pulling more subQ water out to get your skin paper thin. By the time you stop your water intake & have had sufficient carb load & fill out time, you should be standign on stage.
Now for some of the caveats - ts a good idea to test a deplete & carb up way prior to your show day so you have an idea of how well & how fast you respond. As HT mentioned, a fat load worked better for her. I've seen that a few times. Myself, I respond well to a carb up handled in very small amounts and a serious water deplete. But fats bog me down for 2 days. I think the fat thing works for people w/ faster metabolism, but I havent' really dug into that particular hypothesis yet. If the carb up works for you, I highly recommend doing the carb up on a non-water required carb, e.g. potatos instead of rice or oatmeal because the required water to puff up the foodd has to be accounted for in your water depletion. There's also that chance of getting the bloat / stomach ache as well.
Some people also do a carb up on things like raisons, ANPB & jelly on rice cakes, etc. I did that back in 2000, but honestly couldnt' tell you how it worked. The simple sugar will definitely kick your veins up and at that point you are really looking to the fats to fill you out more. The fat load & fat requirements during the final week of prep are still a bit fuzzy to me how they work so I won't go into them. I relied on EFA tabs for my fats on my last days. I also pounded broccoli, but that you also have to be careful of because it can cause some miiserable bloat. (I never leave the house w/o my Beano & digestive enzymes...)
The last item I wanted to address was the electrolyte thingy. For most people the whole water deplete thing is not that big a deal - it is not comfortable being as dried out as you can get (e.g. major cotton mouth) but for the most part, even w/ OTC diuretics, as long as they are potassium sparing, you shouldnt' have any problems w/ an electrolyte imbalance. But, that is the one thing that you hear about w/ regard to competitors passing out on stage - electrolyte imbalance - probably due to overuse of non-potassiuim sparing diuretics (i.e.. NOT OTC...) But, for whatever reason, if you start to feel light-headed while you are standing in the line up before going on stage, it was recommended to me to keep a small bag of potato chips (i.e. salt) w/ you and chew down a couple to help even out your electrolytes if you do feel flaky. Note, I don't mean tired or exhausted, but rather really light-headed.
Other stuff that there are lots of schools of thought on are sodium deplete & load as well as potassium load. I haven't used either so I cant' speak much to them. I'd like to dig more into the potassium load as it worked famously for HT, but dont' know if that is related to her metabolism or it just works.
Hope that helps some
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