As others have said ‘fat burning’ is a bit of a myth.
You lose fat when you take in less calories than you use.
You can take in less calories by eating less/lower calorific food.
You can use more calories by increasing exercise or increasing your base metabolic rate (the amount of energy you need to just exist) You can do this by increasing muscle as it’s a live tissue (unlike fat) that needs energy to exist.
My understanding is that…
Very high intensity exercise causes the body to release hormones to decrease insulin and increase glycogen breakdown from the muscles – to provide glucose – this then triggers the replacement of the glycogen which is quite slow. The high intensity exercise also causes muscle damage which takes energy to repair, and increases muscle strength.
You have to deplete all your glycogen stores before you burn fat – and you’ll bonk/hit the wall first.