A short fast elicits a
"starvation response" increasing norepinephrine, adrenaline, cortisol, growth hormone, thyroid hormones, and glucagon. These hormones increase energy availability and metabolic rate. (r)
You can think of it like this: Imagine having an unsuccessful hunt for a couple of days. The body responds by amplifying systems to help ensure a successful hunt. It will make more energy available to muscles to chase down an animal, it will turn down sleep and turn up alertness, pupils dilate, hearing can even improve.
This increase in energy expenditure (i.e. fat burning) lasts up to 3 days. But after this the body flips the switch and prioritizes conserving energy.
This is where people get mixed up about fasting. More is not better.
Short term, you can have an increase in metabolic rate. Combine this with little to no energy intake and you get rapid weight loss.
However, after 3 days, things turn down. Metabolic rate drops. Muscle tissue catabolizes.
Fasting can be a tool in your
toolbelt, but I'd be cautious of overdoing it.