source: The Oxygen Advantage
BOLT Score is the time it takes your body/mind to respond to you holding your breath. In essence, the first physiological and physical reactions that your brain gives you to start breathing again.
This is better than max breath hold as you can’t ‘will’ yourself to hold it longer, it either is or isn’t.
A healthy BOLT score is 20 seconds for athletic people. The goal of the book is to get you to a BOLT score of 40 seconds.