Introduction
Mahā महा . From mahat (mah) महत् : Great (in space, tie, or quantity); high, vast eminant, important.
Generally known as Upavistha Konasana
Presentation
This is a major posture. It permits a deep stretch of the medial muscles of the thigh. To arrive into the final pose requires slow and progressive practice. In the beginning, the angle may be slow and the hands may have to support the pelvis girdle by resting on the ground behind the back:

Then, during an exhalation, lean slightly forward and during the inhalation bring the arm up vertically while straightening up the back:

Finally, during the following exhalation, lean forward by first tilting the pelvis girdle and then following with the back. With time the hands grasp firmly the toes and the forehead gently lends on the ground.
Note: it is important to not flex the knees. This skews the stretch of the muscles.

The eyelids are closed in Brûmadhya Drishti (glabella point). The tongue gesture is that of Jivâ Bandha or Khéchari Mudrà.
Then, the breathing is applied to permit the circulation of energy. For example, 3/6/6/0. Ujjâyin is suggested as well (throat noise). The inspiration can be visualized starting from the throat and going to the root following the spine. Then, during the full-lung retention, the breathing condenses in the root while practicing an intense Mulà Bandha. Finally, during the exhalation, the prana diffuses equally in the legs and goes back upward through the arms, reaching the throat. And so forth.
In all cases, this posture requires that mulha bandha be constant throughout all breathing phases and more intense during full-lung retention. If the posture exhausts the practitioner, this indicates that the lower lock was not effectively established.
With time, the classical Visamavritti is adopted, following 3/12/6/0 and then 4/16/8/0 and so forth.
A perfect physical posture

Rappelez-vous ceci : le Yoga n'est pas une discipline pour 'Kamikase' mais bien une pratique pour des personnes 'intelligentes' agissant avec ruse et patience.