Crash diets are plans that severely restrict your calorie intake for a few days in hopes that you will lose a lot of weight in a very short period of time. Unfortunately, crash diets promise to much and delivers too little. While they do result in a lot of weight loss, the primary way they accomplish this is by restricting your salt intake.
Salt has the effect of holding water. If you eat a meal that is high in it, you will notice that you gain weight. This weight isn't fat, it's water weight that will leave your body as soon as your salt intake goes back to normal.
By not eating salt for a few days, your body will lose this water weight and you will think the diet has succeeded. As soon as you return to your normal eating habits, the weight will come back and you'll wonder why yet another diet has failed. These crash diets are unhealthy because they basically starve you for a few days.
When you're not eating, you're not getting all the vitamins and minerals your body needs to properly function. The only way to lose weight is through restricting your calorie intake while exercising. This will result in permanent and healthy weight loss.