Siddhasana is a purely meditative posture. It is designed to channelize the prana or vital energy directly to ajna chakra. Shambhavi mudra stimulates ajna chakra so that the pranic impulses coming up from the lower centres can be received. When the chin is placed on the chest, a simplified form of Jalandhara and Ujjayi pranayama is automatically performed. This adjusts the heart rate, blood pressure and brain wave pattern.
When you first start to practice meditation other postures can be utilized, but in the final stages, Siddhasana is the best posture as it enables you to handle the changes that take place in the body during deep meditation. The traditional method of Siddhasana is done in following manner.
Sit with the heel of the left foot pressing the perineum, the area between the anus and the genital organs. Place the right foot so that the heel presses the pubis, directly above the generative organ. Push the toes and edge of the right foot in between the left thigh and calf muscles. Make sure the body is comfortable and steady, spine erect. Lower the chin towards the collarbone, relax the head. Gaze into the eyebrow center- Shambhavi mudra.