Bill Hartman wrote an excellent article on diaphragmatic breathing last month, called
Yoga, Diaphragmatic Breathing Exercises and the Downward Dog
It's an excellent read, as it's got a good explanation of breathing from the diaphragm and how the yoga pose downward dog helps you do just that.
The downward dog variation he shows in the article is a bit different than the Iyengar Yoga downward dog. This is not to say wrong or bad, far from it. But it's different, and it's worth being aware of the differences when you approach the exercise.
No socks. The socks in Bill Hartman's video are probably a concession to the fact that few gyms will let you walk around without shoes, nevermind barefoot. But you do lose some of your ability to grip the floor when you wear socks.
Ground posture vs. Standing posture. The Iyengar posture is clearly a standing posture - you're standing up but with your hands on the ground. The posture in the Bill Hartman article is clearly a ground posture - it even starts with a pushup, and maintains a weight-on-the-hands approach instead of weight-on-the-feet.
Heels down vs. Heels Up. Again, the difference is in the legs and feet. Heels down, straight legs in the Iyengar version, up on the balls of the feet in the Bill Hartman article version.
Both have the same intent and pretty much the same effect on breathing - they line up your torso and (at least theoretically) your internal organs, and let you breathe deeply and from your diaphragm. Try both, especially if your gym will let you ditch the socks and grip the floor.
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