Burning fat for fuel
If it takes from what I read 21 days to start the process of burning fat for fuel,
what happens if you enjoy a meal one day, and eat the white flour products you normally would
Do you start this "21"day cycle all over again??
No it doesn't really work like that. I don't know where these 21 days come from but I geuss it's more of an estimate. Everyone's different. Obviously it doesn't reset from eating 1 meal or even 1 day of high carb.
Your body does already know how to burn fat for fuel, it most likely doesn't do it perfectly yet and needs to adept. Obviously it doesn't just unlrearn it from eating carbs again. If not used for a long time due to low fat however it will obviously become less efficient again. Just like you'll have some trouble riding a car if you haven't for a year.
For me for example, I had no problem with the transition. My normal diet included quite some fat, I always ate anything. For someon that ate high carb low fat for some time it will be different.