My understanding is that that is an old way of thinking that sounds good but doesn't work.

You need to put in the right type of calories: high GI foods will make it harder for you to lose weight by stimulating insulin production which in turns stimulates your cells to sequester all the sugar in your blood and turn it into fat (and you get lethargic and hungry as a bonus). You also need to exercise as your diet continues: your body will slowly burn muscle mass if your intake decreases for long enough, and with less muscle you burn less and eventually hit a wall. Exercise replenishes that muscle mass (assuming you eat enough protein). It's complicated.

I'm not a medical doctor, the above is just my understanding, so take it with a grain of salt.