The answer lies in Siddles and Grossmans writings-but you "don't need to" read those.
"Belly" breathing-note the quotes-is one name given to a breathing technique-also called-autogenic, combat, tactical, meditative, etc.By drawing the belly downand out you engage the diagphram more and increase the amount of oxygen brought into the lungs by utilizing more than the upper third-as most "chest" breathers do.
The technique has been taught to thousand of police and military personnel around the world...and it works. No euphoria or lapses in judgement have been noted.
But what do they know.