SB 4.6.38

kṛtvorau dakṣiṇe savyaṁ
 pāda-padmaṁ ca jānuni
bāhuṁ prakoṣṭhe ’kṣa-mālām
 āsīnaṁ tarka-mudrayā
Synonyms: 
kṛtvā — having placed; ūrau — thigh; dakṣiṇe — at the right; savyam — the left; pāda-padmam — lotus feet; ca — and; jānuni — on his knee; bāhum — hand; prakoṣṭhe — in the end of the right hand; akṣa-mālām — rudrākṣa beads; āsīnam — sitting; tarka-mudrayā — with the mudrā of argument.
Translation: 
His left leg was placed on his right thigh, and his left hand was placed on his left thigh. [This sitting posture is called vīrāsana.] In his right hand he held rudrākṣa beads, and his finger was in the mode of argument.
Purport: 

The sitting posture described herein is called vīrāsana according to the system of aṣṭāṅga-yoga performances. In the performance of yoga there are eight divisions, such as yama and niyama — controlling, following the rules and regulations, then practicing the sitting postures, etc. Besides vīrāsana there are other sitting postures, such as padmāsana and siddhāsana. Practice of these āsanas without elevating oneself to the position of realizing the Supersoul, Viṣṇu, is not the perfectional stage of yoga. Lord Śiva is called yogīśvara, the master of all yogīs, and Kṛṣṇa is called yogeśvara. Yogīśvara indicates that no one can surpass the yoga practice of Lord Śiva, and yogeśvara indicates that no one can surpass the yogic perfection of Kṛṣṇa. Another significant word is tarka-mudrā. This indicates that the fingers are opened and the second finger is raised, along with the arm, to impress the audience with some subject matter. This is actually a symbolic representation.