One benefit of fasting is it uses up your bodies glycogen, sugars, store. When this is gone your body can only burn fat. As you eat you quickly replenish this store.
Your body does naturally lower your metabolism to cope with starvation but keeping active and regular exercise can raise it. Why fasting requires lot of motivation to keep yourself going.
One easy alternative is to take an early tea and not eat afterwards. Make sure get a good 8 hours sleep. When you wake your glycogen stores will be low or out unless you are a mega athlete
Hit the gym/pavement hard before any food. Instant fat burning.
Am only just learning about all the biological processes so could be wrong but I guess even if you look at the 5:2 as a drop in your average calories over a week you will lose weight although its not that quite that simple.