A big part of it is obviously body compisition. These guys likely have very little body fat and so that is where I get lost. Are they losing muscle or can they lose 10 pounds of water weight.
There are 3500 calories in one pound of body fat. The highest rated physical activities burn between 1,500 and 1,700 calories per hour. So it would take approximately two hours of intense activity to lose one pound of body fat. (That is assuming that the person doing the activity is doing so on an empty stomach. This way the body uses body fat for fuel.) These guys are losing a hell of alot more than one pound.
Randy says that he likes to be around 215lbs. and then cut to 205lbs. That's 10 pounds over.

I just found something online. A water gallon to pounds of water calculator.
Results: 1.3 gallons of water = 10.8 pounds. "If you're an adult, your body contains about 40 quarts or 10 gallons of water."
I think we may have found our answer. In theory this method can be used without starving yourself while cutting weight. It also means that once rehydrated you will weigh 10lbs. more than you did at the weigh-in. Also, you will not suffer the adverse effects of starvation and muscle loss. This is basically the most temporary form of weightloss possible. In the end a heavier fighter is a stronger fighter. Thanks for letting me think out loud fellas.