Are you doing yoga when your foot is behind your head, or are you doing acrobatics? There's nothing wrong with a little Cirque du Soleil. I have seen them and they are fantastic! What is important for self knowledge is to make conscious choice, know what you are doing and why. If you're doing yoga it's going to be a completely different experience than doing acrobatics though to the casual observer they appear the same. While fancy postures can take a long time to accomplish, the state of yoga is available to anyone at any time in their process. It's often the case that a newbie or "beginner" will have a spontaneous experience of yoga or oneness and then spend years learning how to find that at will.
Yoga is when everything comes together. Yoga is complete present experience. Yoga is the "cessation of the fluctuations of the mind." Oneness. One can practice cultivating oneness anywhere. Washing the dishes or walking down the street. Start today saying to yourself, "I feel my body standing". Drop down into consciousness, physical awareness, body in space.
So many people find the physical mundane or mistakenly believe that it is unimportant for spiritual matters. This poses a false dichotomy between the physical and the mental. Embracing the physical is actually extremely important. Being in a body is the human experience and you will find it hard to go beyond the physical with out first accepting that.
That is why the poses are so lovely and important. They are a physical vehicle for an out of body experience. They are the cloak that dances as we breath, but they are not the breath. They are the road that leads us to the essence of the thing. It is often said that the whole reason for doing the physical practice of yoga is to make the body ready for sitting comfortably in Podmosana (lotus) for long periods of time as this is the ideal posture for meditation. I think what's even more interesting is when the consciousness can achieve meditation in any posture. This is what occurs with long termed practice. Eventually all poses feel the same since the practitioner is experiencing an internal state of awareness.
The teacher helps the student to understand this by encouraging an evenness through out the physical practice and in regards to the student's reaction to the practice. The teacher guides the student toward stillness and presence during challenging moments helping them to stay aware and focused at all times. What so many don't realize is that this is the practice of yoga. It's happening not only in, but between the postures. A curious potential practitioner comes to observe a class, but what does she look for? Without knowing what to look for we are lost in acrobatics. We must know how to seek out the yoga.
So you think you are doing yoga. I have the greatest job. I get to teach yoga or rather, as I view it, guide students toward an experience of oneness. I ask myself as I watch each practitioner, "is that person doing yoga?" How do I know? As I look at and listen to each practitioner I am working. Even when I'm not adjusting the student or talking to the student, I am working. I am meditating on the student's experience. I am striving to feel and understand. I am internalizing subtle clues based on years of observation and self study. This is the essence of why the western yoga teacher training can't work. It can teach you theory, but not to be in tune. For that you need practice and to be in the presence of a great teacher.
So I leave you with this: When considering a place to practice or a teacher to practice with, remember to look for the yoga. When doing your poses remember to look for the yoga. In day to day life, seek out the yoga. After all, a foot behind the head is just an open hip. The only one who really knows if it's yoga is you.