Holding your breath after exhaling for 15-30 seconds can extend breath-hold duration by reducing the urge to breathe.

“Then after 25 or 30 of those, you exhale all your air. [exhales deeply] You dump all your air. You can do that, your nose or your mouth. And then you hold your breath with your lungs empty for about 15 to 30 seconds. [...] And the reason you can do that is because you've blown off all the carbon dioxide or most of the carbon dioxide in your bloodstream. So you shifted the chemistry of your blood by breathing in a particular way. And by doing that, you are no longer triggering these neurons that cause the gasp reflex or the reflex to breathe.”