Diaphragmatic breathing in laymen terms is "belly breathing." Our diaphragm, the principle muscle of respiration or breathing, separates our chest organs from our stomach organs. Think of it as a an umbrella that opens when we breathe in and closes when we breathe out.
Now take a look at how you breathe in. Does your chest expand? or does your stomach expand? This small test is a great way to indicate if you are properly incorporating your diaphragm while breathing.
Now, we know what it is, but why is it so important, especially for exercise and lifting? If we want to get smart about it, it allows our bodies to go through a full oxygen exchange (i.e. beneficial trade of incoming oxygen and outgoing carbon dioxide). Proper breathing requires less energy and effort overall.
So how does this translate in to your wight-lifting program? Belly breathing teaches you how to properly brace your core for those big lifts and big numbers. Bracing your core creates intra-abdominal pressure that will stabilize your spine by keeping it in the neutral position and set you up from a more than optimal biomechanic set point. A lot of times with trainers or friends they tell you to 'keep your chest up' to prevent injury, which, if you are not a very experienced lifter, will result in you arching your back. This position of your spine does not allow you to create pressure properly and actually can do more harm than good.
Let me give you an example:
You are barbell squatting for 8 reps at 90% RPE. You follow the pop, lock, and drop it method of squats (explained on my Instagram post which I have linked below). On the lock, you take a big belly breath in. As you drop, you imagine pushing your belly into your legs. At the bottom of your squat, it feels like your entire lower abdomen and trunk is full of air. You are in a good position. On your ascent, you slowly let your breath out. Basically, you are fab and have performed a sqwat better than most trainers.
Below, I have put a video of diaphragmatic breathing that you can use to practice with. I have posted it to Instagram as well. Any comments or questions are totally welcome and greatly appreciated.