詳しいガイド（英語）
Meaning and tradition
Surya is Sanskrit for the sun, and the gesture is also called Agni Mudra, agni meaning fire. In the Ayurveda-influenced hasta mudra systems popular in modern yoga, it is taught to increase the fire element and reduce the earth element, and it is considered a heating mudra. It is mainly a modern, popularized teaching, not a gesture from the classical hatha yoga texts.
The logic follows the finger map: thumb is fire, index is air, middle is space, ring is earth, little is water. The ring finger (earth) is folded under the thumb (fire), which these systems read as fire pressing down earth.
How to practice
Sit in Easy Pose or in Chair Pose position on a firm seat, spine tall. Rest the hands palm up on the thighs. Fold each ring finger down so its tip touches the base of the thumb, then rest the thumb lightly across it. The index, middle and little fingers stay straight and loose.
It is often taught with Surya Bhedana, the right-nostril sun-piercing breath, or with Kapalabhati. Both are stimulating, so keep the session short and stop if you feel heat or lightheadedness. Hold for 5 to 15 minutes at most, then release.
Tips and common mistakes
Be gentle. Pressing hard does not add effect and can strain the finger.
Check which finger folds. Unlike Prithvi Mudra, where the ring fingertip touches the thumb tip and the finger stays open, here the ring finger is folded down to the thumb base and covered by the thumb. Unlike Vayu Mudra, where the index folds, and Shunya Mudra, where the middle folds, here the ring finger is the one tucked in.
Keep the other three fingers relaxed and the palms up, both hands matching.
Treat it as a short practice. Because tradition calls it heating, longer is not better.
Tradition vs. evidence
Claims that this gesture raises body heat, aids digestion or burns fat are traditional and have no solid clinical support. What is defensible is the attention anchor and breath pairing; any sense of warmth from slow, focused sitting is ordinary. Do not use it as a medical remedy.