What do you think of my theory?

I'll start with my numbers
205 lbs
17%
6'4"
21 years old

My caloric maintenance(according to a calculator I found online) is 3400-3600. If I were to cut my fat intake completely and only take in 680 calories (my 170 grams of protein) and do enough cardio to burn 680 calories then my body would burn only my body fat in search for energy. Each day on this sort of "fast" I would theoretically be burning 3400+ calories and being in Keto the calories burnt would presumably be body fat. Being that a pound of fat is about 3500 calories that would be a pound plus of fat per day. Would it be unhealthy to do this for one or two days a week with no lifting, just some cardio on those days?

Let me know if you have tried this or have any useful insight.

What you explained is actually the same as the PSMF (Protein Sparing Modified Fast) diet. I hear it cuts through fat like crazy but is hard as **** to do. Search the forums, there should be threads about it.

You will still lose muscle doing that.
It did not take you 1 day to gain the fat, Stop fking trying to lose it in a day.