Mudrā-vidhāna (Lalitopākhyāna): Āvāhanī–Saṃkṣobhiṇī–Ākarṣiṇī and allied Mudrās
सर्वा एकत्र संयोज्य चाङ्गुष्ठपरिपीडिताः / एषा तु प्रथमा मुद्रा योनिमुद्रेति संज्ञिता
sarvā ekatra saṃyojya cāṅguṣṭhaparipīḍitāḥ / eṣā tu prathamā mudrā yonimudreti saṃjñitā
Join all the fingers together in one place and press them with the thumb. This is the first mudrā, known as the Yoni-mudrā.