In the article, they mention that RMR (resting metabolic rate) goes down when your weight goes down, but they don't really explain why, or how crash diets affect your RMR. RMR calories are used for basic movement, body heat, keeping your brain and organs working, etc.

If there's less of you (weight wise), there's less of you to maintain. You carry less when you walk, etc. Also, different types of body (bone, organ, muscle, fat) have different requirements. Muscles and organs take a lot of calories to keep going, and rebuild.

A lot of people when they diet, they lose both fat and muscle. The body will break down muscles as energy too, as well as your fat. This is usually because you're eating less than you expend, calorie wise, and you're probably not strength training.

Lifting weights is a great way to increase your RMR. It takes calories to build and repair muscles after working out, and those bigger muscles also take more calories to maintain. It also forces your body to build up something, preventing the saggy skin you may see on those TV diet shows.

Overall, if you work on building up your muscle, your metabolism will slow less, or maybe even increase, depending on how much you get into it.