A crash diet usually only lasts for a short period of time, in large part because people can only bring themselves to restrict their caloric intake so drastically for so long before they give up and start to eat normally (or start binge eating, which is sometimes what happens following a crash diet).
Crash dieting is never the path to long term weight loss. It is a short term solution to a person’s weight problem and even if it yields results, these results rarely last for any real length of time. Crashing dieting essentially involves starving yourself, or reducing your calories so much for a period of time that you drop a fair amount of weight fairly quickly.
Why can crash dieting sometimes lead to a period of binge eating? Well, there are a few possible reasons for this. For one thing, people who go on a starvation diet tend to get very, very hungry, and when people get ravenously hungry and then they finally allow themselves to resume eating again, they may overeat. Other people follow a starvation period with a stretch of binge eating because they struggle with emotional overeating. That is, they tend to overeat in an attempt to cope with extreme emotional states (such as extreme sadness or extreme happiness).
For people who engage in emotional overeating (either in general or in the aftermath of a crash diet), it can sometimes be helpful to speak to a therapist and/or to start attending Overeaters Anonymous meetings to try to get to the bottom of why they engage in this behavior and figure out ways to break out of this self destructive behavioral pattern.