alternate day fasting(ADF): 24 hours eating/24 hour fast repeat
eat stop eat: one or two 24 hour fasts per week on nonconsecutive days (basically a less aggressive alternate day fast)
intermittent fasting(IF): train fasted, have your biggest meal post workout and you have an eating window 4-8 hours long.
what to eat during a fast? stimulants, water, fiber, anything with no calories. you can toss in some vegetables when you first start to make it easier, just dont cook them with butter or something, that defeats the point of fasting.
what are they used for? all of them are used for burning fat. you can toss in a day or two of fasting per week on a bulk and turn it into a lean bulk. how long can they be used for? forever, the secret is that as long as your still hitting decent macros in IF you can maintain your metabolism. with ADF, you reset your metabolism every other day when you eat so your metabolism never slows. you can get down to pretty low bodyfat by using these methods, with ADF fat loss of 2lbs per week is very much doable without losing muscle. your weight will zig zag tho from a lack of carbs on fast day and you will lose a lot of water weight. if you dont eat enough carbs on your eat day you may start to look flat after some time, but all you have to do is skip a fasting day (eat 3 days in a row) and you get a glycogen supercompensation effect so u look even bigger.