I read the same book, and while it did help me in several respects (nasal breathing, mandibular and maxillar expansion via an orthodontic device, mouth taping), I only used its box breathing techniques under acute stress. It wasn't until I read a chapter in Peter Attia's Outlive about breathing techniques that I started changing my breathing in a way that, for me, feels like it's actively stimulating the vagus nerve. Here's how it works.
You want to breathe to expand your entire diaphragm, breathing into your upper and lower chest. One way to do this is to breathe through your nose slowly enough that you can't hear the sound of your own breathing.
To get a hang of it, lay on your back on a flat surface, place a hand on your stomach and a hand on your chest, and breathe in slowly. You should feel your stomach and chest rise evenly, neither too much in the stomach, nor too much in the chest.
Once you understand this feeling, you can practice it throughout your day. You feel your whole diaphragm expand, feeling the tension through your chest, stomach, and even expanding into your lower back. Anyways, that's how it's described in the book. I can say that since starting this breathing technique, I've felt a lot calmer. Best of luck, check the book out if you'd like a far more detailed and accurate explanation than mine.
You want to breathe to expand your entire diaphragm, breathing into your upper and lower chest. One way to do this is to breathe through your nose slowly enough that you can't hear the sound of your own breathing.
To get a hang of it, lay on your back on a flat surface, place a hand on your stomach and a hand on your chest, and breathe in slowly. You should feel your stomach and chest rise evenly, neither too much in the stomach, nor too much in the chest.
Once you understand this feeling, you can practice it throughout your day. You feel your whole diaphragm expand, feeling the tension through your chest, stomach, and even expanding into your lower back. Anyways, that's how it's described in the book. I can say that since starting this breathing technique, I've felt a lot calmer. Best of luck, check the book out if you'd like a far more detailed and accurate explanation than mine.