In every aspect of creation, you will observe polarity. For every positive there exists a negative — day and night, white and black, sound and silence — together they make a whole or poorna. On Shivratri, Shiv and Shakti, two opposites, merge, representing poornta (completeness) in the brahmand. Shiv and Shakti pertain to the manifest creation (vyakt), they emerge from the primordial element that is unmanifest (avyakt). Only once the merger of Shiv and Shakti is complete can one realize the unmanifest, which is the purpose of Yog and Sanatan Kriya.
Shiv is the karta or the doer. It may be likened to a vehicle. If you remove ‘i’ from Shiv, it becomes ‘shav’ (corpse). The ‘i’ representsShakti, which is the fuel that energizes this vehicle to perform action. Shakti is the force of the physical world. Without the merger ofShiv and Shakti everything is incomplete.