In the Vajrayana Buddhist tradition Istadevata (Yidam) are called mind bound deities. Because the nature of these deities is the same as the nature of mind. The practitioners in Vajrayana Buddhism visualize Istadevatas such as Hevajra, Cakrasamvara, Kalacakra and others during developing stages of meditation (skt. Utpattikrama) as their Samatha and Vipasyana practice. They are originated from their seed syllables which appear from a non-dual state of mind, free of thought constructs or state of emptiness.They are not mere imaginations. They are the visualized forms of enlightened minds. They are all Sambhogakaya Buddha forms emanated from a true mind. These deities have however, no real existence as such. But they are the emanated forms of enlightened mind.
Practitioners who have an affinity to these deities in one form or the other can develop their realization very quickly with these practices since these deities represent emotional inclination of the practitioners. If some practitioners like to have Avalokiteshvara, the other may opt for Arya Tara. Some practitioners of aggressive nature rather prefer wrathful deities to meditate or to identified with. In all the Tantric Buddhist tradition these deities are visualized clearly with different colors, forms, faces, hands, all with different ornaments.
The purpose of this deity yoga is to take the form of the deity and actualize his Buddha activities during meditation. This creative visualization method is used to achieve a form body endowed with 32 major marks and 80 minor marks i.e. Sambhogakaya. It is said that the fruit of Buddhahood must be achieved through a method which has the form of the Buddha.
One of the characteristic features of the Yidam deity is the sexual imagery found in Highest yoga tantra (Anuttara tantra). Upon confrontation with father-mother union (Yab-yum) aspect of the Buddha, a casual observer may find it difficult and astonishing to understand the fearsome and erotic nature of the images in the context of Vajrayana Buddhism.
In Nepal this Tantric knowledge is said to be guarded by Nepalese Vajracharyas and in Tibet, by Tibetan Gurus. These Highest Yoga tantra deities are kept strictly secret and are to be seen by the initiated only. However such deities, which were kept secret in past, are now finding their way into the open for different reasons and consequently, the meanings or implication relating to these deities have been the subject of growing curiosity.
It is true that Vajrayana is the vehicle of ‘skill in means’ which utilizes even desire or lust to attain Buddhahood. Vajrayana being the path of transformation utilizes even sexual bliss to realize emptiness or nature of mind.It is said that these practices of Highest Yoga tantra deities involving sexual imagery are appropriate for the most gifted practitioners only. Through such creative visualization process they develop experiences of emptiness of the highest standard, that is, Buddha-nature.
The examples of such tantras are Guhyasamaja, Hevajra, Kalacakra, etc. Highest Yoga tantra claims that Buddhahood is possible within a single life time. Eighty-four maha-siddhas of India, Nepal and Tibet attained enlightenment in their own life time through the practice of such tantras.