Placing the ankles below the testicles, on either side of the perineum, with the heels firmly positioned — this is known as Vajrāsana.

Vajrāsana (thunderbolt/diamond posture) has two interpretations in the tradition:

1. Haṭha Yoga Pradīpikā version — Seated on the heels with pressure on the perineum, similar to what we now call a modified Siddhāsana.

2. Common modern version — Kneeling seated on the heels, with the tops of the feet resting on the floor.

Vajra means both “thunderbolt” and “diamond,” symbolizing:

• Hardness — The posture is stable and unshakeable
• Energy — Vajra-nāḍī is the central channel within suṣumṇā

The reference to sīvanī (perineum) indicates that this posture, like Siddhāsana, has an energetic purpose: pressure on the perineum stimulates mūlādhāra cakra and helps direct apāna vāyu upward.

Note: This traditional posture differs from the Vajrāsana commonly taught in modern classes.