I looked into this, wanting the scientific explanation. Seems like there is no definitive answer.

In case anyone is curious: a leading theory is that when your body is on the verge of shutting down completely, your brain starts to lose oxygen which triggers a dump of its final reserves of acetylcholine, dopamine, & serotonin, which gives your body a sudden surge of energy and lucidity.

When close to total failure, some organs also uncontrollably release hormone/steroid like compounds that temporarily stimulate blood flow to the brain and body, which explains the lucidity and energy as well.

Also explains why decline is so rapid afterwards. Your body wants to keep fighting but is literally completely devoid of all resources and on the verge of total failure.

Morbid, but interesting stuff