大毘盧遮那成佛經疏 13.187

13.187

次金剛月黶忿怒印。此金剛在佛額豪相而生，故以為名。豪相明淨猶如滿月，故號月黶也。如前五股金剛印，屈二風如勾而捻二空，令少屈不相捻著，即是也（與前異也）。

Next is the Vajracandratilaka Krodha mudrā. This vajra is born from the ūrṇā on the Buddha's forehead, hence the name. The ūrṇā is luminous and pure like a full moon, hence called 'moon-spot'. As with the previous five-pronged vajra mudrā, bend the two wind fingers like hooks to pinch the two space fingers, causing them to bend slightly without touching each other—that is it (it differs from the previous one).