Two forms of energy.
Pravritti is akin to the yang. The Shakti energy plunges downwards and produces the world of forms. It is the material.
Nivritti is yin. It moves in an inward direction from the gross to the more subtle.
These energies correspond to different approaches to the spiritual path.
Tantra says first pravritti, exhaust and fulfill all interest in things of the world and one will naturally progress to Nivritti with no force or effort required. Nivritti leads to experience of the infinite.
The way of the world, and the path of return.