By dehydrating. Almost all fighters; from wrestlers, to MMA fighters, to boxers do it. The body, by weight is 65% water. You can easily lose 2 liters of water weight in an hour.
Do the math. A lot of water is held within muscle tissue.
Who's to say they aren't affected?
Dehydration reduces blood plasma, and plasma volume, decreases testosterone levels, reduced muscle blood flow...and many other detrimental effects.
Depending on how much the fighter is cutting, most of the weight will be lost during training. Usually 10lbs. or less remains, and that can easily be cut with a few hours in a sauna suit.
They aren't dropping the entire, let's just use for an example 20 pounds before the weigh in. The final dehydration begins the week of the weigh in, and almost all fighters significantly lower carbohydrate intake, which in turn causes the body to store much less water, and expel less water in the process.
It actually is quite common. I use MMA fighter Georges St. Pierre as an example because he cuts around 30 pounds to make the 170 limit. He is an even better example to use because his muscle is so dense...comparable to Manny Pacquiao.
Its all water weight, and these guys have experienced trainers and nutritionists who have dehydrating and rehydrating down to a science.
How does it seem suspicious?
Most fighters in these weight classes rehydrate to similar weights. Find the glaring difference in the physical statures and weigh in weights, and you'll see that there really aren't any.
O.k. I was asking questions hypothetically; I know enough about losing and gaining weight naturally to know when something does not seem right. First, let's get this out of the way: George St. Pierre walks around at 185, not 200. See link http://www.mmanews.com/ufc/Transcript-Of-Exclusive-Georges-St.-Pierre-Interview.html.
Now, Pacquiao at 144 is already in state of dehydration. We know this because for the Cotto fight he weighed, fight night, 149-150. And we all know that a fully hydrated individual does not put on 5-6 pounds with just food. Let's also start by noting that 5-6% body fat is about as good as it is going to get for a natural man. This is also a body fat level for which one must train and diet. There is no such thing as being born with genetics that allow you to stay at 6% body fat all the time. We also know that the closer you get to this number, the more difficult it becomes to achieve more body fat loss. You seem well versed in biology and physiology, so I don't have to tell you that.
Pacquiao weighs naturally 150 pounds at 8% (12 pounds of total fat) body fat. He starts to diet and train to make 129. In the weeks approaching the fight, he trains and diets hard to lose 5 pounds of fat; unfortunately, he loses 3 pounds of muscle in the meantime (muscle catabolism -wasting, it sucks, but it happens). He now weighs 142 with 5% body fat, extremely ripped and hard. He has already lost significant amount of water, but he's not done, he's going to lose an additional 13 pounds of water (9% of total bodyweight at this point) the last week before the fight. He does so successfully, but not before stopping by the hospital....
Come on! Not even you believe that is possible.
The trip down from 140 is not that difficult and is what most people would expect. Pacquiao out of shape at 150, loses 10-12 pounds of fat, water, and some muscle, and then loses the additional water weight to make 130, looking rock solid. It is a journey of 20 total pounds, but starting from 150 and out of shape. It makes sense. So...where did those extra 10 pounds of lean mass come from? That is what has everybody in a frenzy my friend.