The pose shown for Ardh Matseyandra asan is wrong. The pose shown is for Vakrasan, which is to b practiced before trying Ardh Matseyandra asan. Also Ardh Matseyandra asan does not mean 'half spinal twist'. Actually Matseyandra Rishi used to sit in a pose, which is Matseyandra asan, but it is very very difficult to do, hence Ardh Matseyandra asan is practiced by those who cannot do Matseyandra asan. In Ardh Matseyandra asan, there is twisting of spinal cord, pressing of stomach with hand & leg, rotation of head etc.
Yogasans & Pranayams r very good for mental & physical health. Simple yogasans can b done by anyone by reading some books or seeing television, seeing videos etc. But for difficult asans as well as Pranayams, some guidance should b taken from trained Yogacharyas.
