I tried it for about a 6 weeks. The idea is simple. If your body doesn't have any carbs to burn it will burn fat instead. I lost about 12lbs in that 6 weeks.
A lot of that weight was probably water weight, not fat. Carbs are stored in your muscles as glycogen, and each gram of carbs holds on to about 3-4 grams of water. When you restrict carbs, your body will use that glycogen as fuel. Hence the rapid weight loss. If you want to see if that weight loss was fat, eat a pizza or bowl of pasta and see if you are still down 12 pounds the day after.
The idea is simple. If your body doesn't have any carbs to burn it will burn fat instead
Your body will still burn dietary fat first. If you are in a calorie deficit, then your body will move into burning stored body fat, but it would do that on a carb based diet as well.
Keto does not have any magical fat burning properties. If you eat an excessive amount of calories on keto you will still gain weight. Keto does have some appetite supressive effects, so any success with keto comes from indirectly restricting calories.
Science, out!