Tantra, or Secret Mantra, has four levels:
Highest Yoga Tantra
Action Tantra principally emphasizes external actions, Performance Tantra places equal emphasis on both external and internal actions, Yoga Tantra principally emphasizes internal actions, and Highest Yoga Tantra is the supreme class of Tantra.
All four levels of Secret Mantra transform great bliss into the spiritual path, but the methods of transformation differ according to the level being practised.
In Action Tantra, the meditator generates bliss by looking at a visualized goddess, and then transforms that bliss into the path. In Performance Tantra, the meditator generates bliss by exchanging smiles with the goddess, and in Yoga Tantra, by holding hands with her and so forth.
In Highest Yoga Tantra, the meditator generates bliss by imagining sexual embrace with a consort and, at advanced stages, by engaging in actual embrace; and then transforms that bliss into the spiritual path. It should be noted, however, that it is very difficult to use great bliss as a method for attaining enlightenment, and if we are able to do so we have indeed attained a formidable accomplishment.
As the great Mahasiddha Saraha said, ‘Everyone is excited by copulation, but very few can transform that bliss into the spiritual path’.
For more information, see Tantric Grounds and Paths.