A crash diet is a diet with a very large calorie deficit, meant to take off as much weight as possible in a short time. Typically calories drop sharply and suddenly, for example after the holidays or before an event.
Why it looks so tempting
In the first week of a crash diet the scale often drops impressively. Much of that is water, though: with fewer carbs your glycogen stores empty, and glycogen is stored with about three to four parts water. You would have lost that water on a moderate diet too. If you mistake it for fat loss, you learn the wrong rate.
What it costs
For strength athletes, losing about 0.5 to 1 percent of body weight per week is considered a good range (Helms et al. 2014). Beyond that, the risk of losing muscle along with fat goes up. In a study of elite athletes (Garthe et al. 2011), only the group losing at the slower rate of 0.7 percent per week gained lean mass, while the group losing 1.4 percent did not. A meta-analysis by Murphy and Koehler 2022 found that a deficit of about 500 kcal a day or more already prevented gains in lean mass.
Add hunger, worse training performance and a bad mood. The leaner you are, the more these downsides weigh.
The alternative
Instead of halving your calories, pick a rate in the range above, eat plenty of protein and keep training heavy. If you deliberately want to lose faster, keep it time-limited, as in a mini cut. Whether your rate fits shows in your trend weight after two to three weeks, not in the drop of the first few days.