Shakti, power is the flow of energy between the Vishnu and Shiva back and forth.
Usha is that which keeps getting replenished. ….the rays of the sun ; the flowing rivers ; the sprouting leaves are all examples of this form of Shakti. The reservoir of energy is so huge that we get a feeling of ‘continuity’ in using it. Literally Usha means ‘the dawn’ and Rig Veda, the oldest of the Vedas, is replete with its praise.