It depends on the person and biochemical pathways. Too complicate to explain it here.
Most food even meat contains trace of carb.

Human body can make carb(glucose and lactose).

We burn fat from your body fat storage or fat in the blood stream(food intake) when we are in lipogenesis-less. In certain pathways, some carb does help fat burning, but don't bet on it.

In theory, when a low carb person consumes carb, it goes to burning first.
Next, excess carb will go to glycogen. If you have lots of muscle, it will get you 2 to 10+ lbs of glycogen(if you do not consume excess fat at the same time.)

So go back to zero carb will burn up your glycogen in 1 to 3 days.

Glycogen acts like a safety buffer.