When people talk about weight loss they sometimes conflate weight cutting and weight loss. Let me explain.
In many sports competitors are divided into weight classes. MMA is a good example - there are divisions, and if you watch the fights the ring announcer always says something like, "weighing in at 155 lbs. in the blue corner, it's..." and so forth. You may think that the guy in the blue corner actually weighs 155 lbs., but in fact the MMA weigh-ins (the event where the fighters actually get up on a scale and have to weigh in at or less than the maximum for their weight class) is at least 24 and often 28 hours before the fight. What does this mean?
Well, imagine a fighter, Adam, contracts to do a lightweight bout (155 lbs. or less) on April 1. On January 1 Adam probably doesn't weigh 155 or anywhere near it - he probably weighs around 185 or 190 lbs (as does his competitor). He's probably a little pudgy, so he's not a super lean and cut 185, but nowhere near 155.
Adam then starts training intensely for his fight. He might train 2 or 3 times a day, depending on whether he's a full time fighter or not. Over the next 12 weeks he'll lose some fat as he trains hard and starts watching what he eats. His weight will fluctuate a lot over each day, because in a 2 hour workout he might drop 6 lbs. of water, but in general his weight will drop. After 12 weeks, a few days before the fight, he might weigh, when fully hydrated and after a meal, something like 170 lbs.
Where do the next 15 lbs. of weight loss come from? Here's the trick. Starting a couple of days before the weigh in Adam will start to dehydrate himself. He'll eat very little or no food. He might spend long hours in a sauna or slowly riding an exercise bike to sweat out fluids. He won't drink very much. He won't eat any salt. In general, he'll deplete his glycogen stores as fully as he can, then sweat out the remaining pounds. This is called "cutting weight."
Naturally, while Adam is dehydrating himself, he'll lose a little bit more bodyfat, along with some muscle, because not eating while sitting on an exercise bike is going to burn some tissue, but most of the 15 lb. weight loss at that point is coming from water. Then, after weighing only 155 lbs. at the weigh-in, Adam will re-hydrate himself, drinking gallons of Gatorade and Pedialyte, eating some solid food and some salt, restoring both muscle glycogen and the associated fluid.
Why do this? Well, by losing and then replacing all that water Adam can step into the ring at 170 lbs or maybe a little more. If his opponent isn't cutting weight - if his opponent is just a guy who, when in shape, weighs around 155 lbs. - then Adam will have a huge advantage in the fight. That isn't generally what happens, at leat not at the higher levels of the sport. At the high levels of the sport all the athletes cut weight, and the two guys in the ring for a lightweight bout (155 and under) usually both weigh around 165 or 170 lbs. at the time of the actual fight.
Fighters aren't the only ones who do this, of course. Bodybuilders and fitness models will dehydrate before a contest or photo shoot to appear more cut. I'm sure athletes in other sports use water manipulation as part of their peaking procedures.
Why am I bringing this up? The dehydration - rehydration part of this process is useful for professional athletes trying to compete within a weight class, but it has NO PART of an amateur athlete's fat loss or weight loss repertoire. It's unhealthy, dangerous, and has little to no long term benefit. In other words, dehydrating yourself on purpose to hit a certain scale weight won't help you lose fat and improve your body composition. In fact, it will make it harder overall to keep your body in good shape. There are also acute dangers to dehydration - athletes die from dehydration, I wouldn't say all the time, but it happens. Plus, you can't actually perform well while dehydrated, so it's not going to make you a better fighter - you're going to have to rehydrate before a fight or sparring session.
If you watch shows about MMA you'll see guys sweating or sitting in saunas to make weight, but that has nothing to do with getting leaner or with losing fat, only with dehydrating themselves to make a weight class. Don't ever do it yourself unless you're in that exact same situation. I'll refer to healthy fat loss as leaning out and to dehydration as cutting weight if I write about this again.