You should not be tilted sideways, backwards, or forwards. You should be sitting straight up as if you were supporting the sky with your head. This is not just form or breathing. It expresses the key point of Buddhism. It is a perfect expression of your Buddha nature. If you want true understanding of Buddhism, you should practice this way. These forms are not a means of obtaining the right state of mind. To take this posture is itself the purpose of our practice. When you have this posture, you have the right state of mind, so there is no need to try to attain some special state.