大毘盧遮那成佛經疏 13.24

13.24

即次作吉祥商佉印。先作空中合掌（如前），屈二空指，以二風指壓之，令如商佉形。作印已，即近口吹之，如吹螺之法也。此是滿一切願吉祥法螺印。由作此印故，即滿一切善願，宣說大法能令十方普得聞知。此即是寂靜涅槃印也。其真言歸命同前。

Next, form the auspicious śaṅkha mudrā. First form the hollow añjali (as before), bend the two space fingers, and press them with the two wind fingers to form the shape of a śaṅkha. Having formed the mudrā, bring it close to the mouth and blow into it, like the method of blowing a conch. This is the auspicious Dharma-conch mudrā that fulfills all wishes. By forming this mudrā, all wholesome wishes are fulfilled, and the proclamation of the great Dharma causes the ten directions to hear and know it universally. This is precisely the mudrā of tranquil nirvāṇa. Its mantra's homage is the same as before.