Separate from body, it cannot achieve.
Body also is like a wall;
Separate from mind it is without activity.
Similarly the yoga of seals....
-Shriparamadhya
Mind separate from body cannot openly display activities; body separate from mind is the same. Rather, actions must be done upon the aggregation of mind and body. Just so, here when practicing techniques for transforming basic--or ordinary--body, speech, mind, and activities into those of the fruit stage of Buddhahood, it is necessary both to cultivate (1) internal meditative stabilization on a divine body, on a vajra on a moon disc at the heart, on a seed syllable on a vajra in the throat, and on a vajra at the heart, and (2) at the same time to construct the appropriate gesture, or seal, with the hands. These must be done in unison, for it is said in Yoga Tantra ritual texts that if you fail to construct seals with the hands, the rite is nullified. Unlike on other occasions, it is not just that if the hand-seals are constructed, it is better, but if not, there is no fault of nullifying the rite. Here, they must be done.
--from Yoga Tantra: Paths to Magical Feats by H.H. the Dalai Lama, Dzong-ka-ba and Jeffrey Hopkins, translated and edited by Jeffrey Hopkins,
