Have you noticed how in order for a workout to be any good these days, it has to be “intense" or “killer"? Diaphragmatic breathing - slow, deep breaths that fill your belly - isn’t sexy. It’s not killer or hardcore or badass, but it's a necessity for health and performance. Think about it; breathing is the very first thing you do when you’re born and it’s the last thing you do before you die. It must be important. Really important.
It balances out your sympathetic and parasympathetic nervous systems, allowing your body to function more optimally. It helps reduce the stress hormones adrenaline and cortisol, which are more often than not elevated due to stress-filled work environments and a lack of sleep. It's also been reported that diaphragmatic breathing:
As you can see, there are some important benefits to regularly practicing diaphragmatic breathing. You can do it to replace a workout when you're overly stressed, or you can do it at the end of your workout.
Here's how to do it
Pick a comfortable position to lie down on your back, close your eyes, inhale slowly and fully into your belly so that you can feel it rise then exhale slowly and fully so that you can feel your belly sink back down. Try counting to 4 or 5 on the inhale and again counting to 4 or 5 on the exhale. Do this deep belly breathing 3-5 times, and then just completely let go and breathe normally for the next 5 minutes or more. You’ll be surprised by not only how good you feel after you’ve done it, but by how well you'll sleep that night and how recovered you'll feel for your next workout.
If your mind is wandering all over the place (as it tends to do), just bring your attention to the breath - noticing how it feels as it enters and exits your nostrils (cooler on the way in, warmer on the way out) and what is happening to your belly (rising and falling). Focus on completely and totally relaxing your body, especially the face (in particular the eyebrows and eyes) until it feels like your whole body is melting into the floor.
Most importantly, be kind to yourself. Don't worry if your mind remains busy. The point is that you're fully relaxing your body and breathing.