How to practice

Inhale to lengthen, exhale to rotate; grow tall first, twist second.

Sit with the legs extended; bend the right knee and step the right foot outside the left thigh.
Bend the left knee and bring the left heel toward the right outer hip (or keep the left leg extended).
Inhale and lengthen the spine; exhale and rotate toward the right, right hand behind for support.
Hug the right knee with the left arm, or hook the left elbow outside the knee.
Turn from the belly upward, neck last; hold 5–8 breaths.
Release on an exhale and repeat twisting to the left.

Benefits

Rotates and hydrates the whole spine
Stimulates the abdominal organs, liver and digestion
Opens the outer hip and glutes
Traditionally credited with kindling agni and awakening kuṇḍalinī in the haṭha lore

Contraindications & cautions

Recent spine injury or surgery, wait for recovery and a teacher's guidance
Pregnancy, practise an open, gentle seated twist instead
Severe disc problems, keep the twist small and both sit bones grounded

Skip or modify the pose on days a caution applies to you, and work with a qualified teacher for injuries, pregnancy or health conditions.

In the tradition

Matsyendranāth, the fish-bodied sage of tradition, is said to have overheard Śiva teaching the deepest yoga at the bottom of the ocean. The pose named for his seat is the haṭha canon’s great twist, traditionally allied with strong digestion and an awakened spine, friendly to kapha and gently rearranging for vāta.

Sources & further reading

Haṭha Yoga Pradīpikā , Svātmārāma (chapter 1, verses 26–27; the teaching seat of Matsyendra among the chief āsanas)
Light on Yoga , B. K. S. Iyengar (Ardha Matsyendrāsana I)

Classical citations mark where a tradition is drawn on; modern books and videos are shared as further guidance, not as prescriptions.