You don’t really need the gimmick black box to do that, although it probably will help some people manage their breathing rhythm. My brother can “manually” control his breathing to noticeably lower his BP on a subsequent test — not sure how long the effect lasts. I don’t seem to have the self control. But since my bp meds are $4 at Walmart I have no big incentive to invest in a method like that (my guess is many people use the device for a few days and then give it up).
No, I would think counting "a thousand one, a thousand two, etc." to 1003 (for inhaling) and 1004 to 1006 (for exhaling) would do the trick (if there's a trick to be done). And just to try it seems harmless -- no unpleasant side effects. etc.