Tib., thabs dang shes rab (thabs shes kha sbyor)
Skt., upaya
Tib., thabs: Skillful Means, Method
Skt., prajna
Tib., shes rab: Profound Cognition, Wisdom, Knowledge
Not only can the Tibetan thabs dang shes rab be translated in several ways - means and knowledge, skillful means and wisdom, compassion and emptiness - the exact interpretation and meaning given to the phrase is different within various schools. Most simple is the general view in which prajna (ye shes) is seen as the knowledge of realizing egolessness and upaya (thabs) as the method or technique that brings about this realization; a most necessary step for attaining Buddhahood.
In Dzogchen, thabs dang shes rab is related to the Trekchö and Tögal practices of the Secret Precept Class (Skt., upadesa; Tib., man-ngag gi-sde). Here, prajna/knowledge represents one's view of primordial purity (the Trekchö practice of realizing enlightenment in the present moment), while upaya/means is the meditation of spontaneous being (the Tögal practice of exhausting all defilements and fixations) through which the rainbow body can be realized within one lifetime.
In yet a different view again, according to the Kagyu teachings, upaya/means refers to the so-called "Path of Means" (i.e. the Six Doctrines of Naropa), and prajna/knowledge to the "Path of Liberation" (the practice of the Four Yoga's of Mahamudra).
However, all these deeply philosophical and highly innocent sounding expressions have also another meaning, where thabs dang shes rab is a way of referring to the (sexual) union between male (skillful means) and female (profound cognition) adepts. This meaning or content would be known and understood only by initiates of a certain degree.
Occuring in many a Tantric text, the phrase is a very good example of the coded/guarded terminology often used to transmit secret techniques based on sexual energy and practiced with appropriate rituals; sometimes on a mental plane (i.e. visualized) and sometimes on the physical plane (with a selected consort, a sangyum). This can also be clearly deduced from the many variations of terms centered on thabs lam, the Path of Skillful Means.
As if to underline the above, both the terms skillful means and bliss are also found in the description of male yidam; as are profound cognition and emptiness for the female ones.
Very much the same is indicated by the concept of Bliss - Emptiness referred to within the context of terma revelation where bliss represents the male, emptiness the female. The combination and union of the two enables the initiate or a terton to achieve the lucidity necessary to comprehend whatever visionary experience he/she may undergo.