I'm using the popular MAF definition; Aerobic = primarily glycogen-fueled, anaerobic = primarily fat-fueled. .
Somewhere in there, Maffetone describes the process whereby a little "charge" of glycogen is necessary to "spark" the burning of the fat (I made up the cool terms myself because I forget how he really describes it). Basically, though, fat doesn't burn on its own, it needs a little fuel to get it going. ... Somehow or other ... I do get lost when I start trying to understand this on the molecular level.