starvation mode

i find it confusing this idea of "starvation mode", sure maybe your body tries not to burn calories as much but if you force it to with exercise or a busy lifestyle it'll have to burn the calories anyways right? like it can't just pull the energy you need out off the air.

also after 48 hrs glycogen in your muscles gets burned away, and from that point your burning your fat reserves only.

any opinions? thanks