Weight cutting through dehydration may not only increase risk of concussions, it may lead to long term neurological deterioration affecting athletes in more areas than previously thought
(see article below for details)
In the realm of combat sports, athletes are paired off against opponents of the same size. This is obviously done for fighter safety. Having a 220 pound wrestler grabbing and throwing a 140 pound opponent would be dangerous for the smaller athlete. Hours before competition (in jiu jitsu and wrestling tournaments) or a day to a day and a half before competition (in kickboxing, boxing and MMA fights) athletes have to be weighed in order to make sure they are at their competition weight (or below, but never above). Theoretically this keeps fighters safe.
Unfortunately, this may not be true. As can be expected, athletes are doing anything they can to shrink their body weight as low as possible in hopes of fighting other athletes that are smaller than them. This is usually achieved by dehydrating the body severely in the days leading up to a weigh-in. Fluids can be replaced rather quickly and therefore, the athlete who can drop the most water in a short time will be the more muscular and heavier athlete in competition. In a real sense, there are two events happening: a weight cut competition and a fight.