Strengthening the breathing muscles is what improves the blood pressure, according to the article.
Now I'm going to get one of these things!
In yoga there is a practice called Ujjayi Breathing where you restrict airflow in and out of the lungs via the the throat, making a soft sound. This is often combined with meditation or sometimes uses in very energetic situations to calm the breath.
Strengthening the muscles you use to breathe make it easier to do and thus puts your body in a relaxed state more often.
You don't need a device to do this. You can also do any popular deep breathing exercise like Wim Hof, square breathing, 4/7/8 breathing, or restricting airflow with thumbs in nostrils. A device may be more effective at training your resistance however.