Pradosham is the time when Lord Shiva drunk the Aalahala poison that emerged out during the churning of the milky ocean. The poison threatened to destroy all the worlds and everyone in there. So out of deep compassion, Lord Shiva inorder to save the Universe consumed it and danced in bliss in the form of Nataraja on the head of His divine bull vehicle Nandi between 4.30pm – 6 pm on the thirteenth phase of moon. that is the triyodashi thithi.
So every thirteenth phase of moon evening Lord Shiva radiates the energy to destroy the negativity thereby saving us and protecting us.
Also on the Pradosham time Shiva delivered the secret meditation techniques for Enlightenment in the book Vijnana Bhairava Tantra to Devi.
To get rid of all negative influences from our lives and to get all our prayers fulfilled and to please Lord Shiva and to head towards Enlightenment, Rudrabhishekam is offered Shiva Linga. Chanting Shri Rudram while doing the Abishekam during the pradosha time nullifies all the negativities and the planetary afflictions. The energy and the vibrations released during this pooja is so immense and powerful so that all the devotees are requested to darshan or see Lord Nataraja with his consort in between the two horns of the Nandi Devar who is the vehicle of lord Shiva.