Ashtanga Yoga asks your body to do things it's never done before. If you've never stood on your head, the practice asks you to stand on your head.
The practice asks you to bend your body in ways you never thought was possible, this is important because it's only at the limit that you believe possible for yourself and your body, that you know who you truly are, and it's only at the limit of what you know yourself to be, that your nervous system gets stressed.
You need to challenge the limits of what you know you can and cannot do in your practice because in that moment you're breath will be challenge, and in that moment your yoga starts.
We use our yoga practice to manufacture a false stress, so that it triggers the same neurological response you would normally experience under deeply stressful situations, and instead of screaming, running away, or freaking out, the practice asks you to breath, stay focused and stay in the posture.
This helps us to control our breath and our nervous system not only on the mat, but off the mat as well. This is how we learn to stay calm in stressful real life situations.