Rinsing your mouth with sugar water is known to help with running performance. It sounds like the same mechanism. I remember recently reading a longer article about the running thing, where swishing the solution in the mouth improved the times even for professional athletes, who were probably already pretty motivated and experienced at going all out. So it seems likely that whatever mechanism is at work here is pretty low-level and is doing stuff a bit beyond what's easily accessible with just trying really hard.

Wonder how hard it would be to control the involved pathway more directly than with the sugar water thing. Just make the brain constantly think that it's okay to go all-out with the willpower, there's lots of fuel available if we need it.

Also, aren't there theories floating out that the placebo effect is something similar? The brain is blocking top performance when it's unsure about the energy reserves, and it blocks the most efficient healing when it's unsure about whether there's a nice, long-term safe place where you can go on the mend and just sleep and eat a lot instead of foraging for food.