“For “the interval that it takes to breathe…”, it probably has to be during a formal sitting.” – You might be right.
I practiced it for a while when i went through Kashmir Shaivism. By the way: Sadhguru says that Buddhism and Shaivism are alike…weird isn’t it ?
Back to our subject: Shaivas say that the Paramasiva dwells in that pause between the end of exhaling and the beginning of inhaling.We of course we say differently: that which might dwell there is the…Deathless ? Just asking.
Such focalisation > absorption method should be practiced only via formal sessions.It seems easy but it’s not, because of the subtlety of the passage. Inhaling: rise, Exhaling: fall (anicca, thus…); the interval : the deathless?
An historical aside/curiosity not directly related to your quest :alot of Indian/Asian meditation techniques it’s very difficult to know who started doing what when.
The info above i’m conveying it to you as a curiosity; for serious practice you should , as Lal advised , focus on Thilakhanna, which contains , i’m convinced, the essence of true dhamma.