Shiv is often described as adi (unborn), anant (endless), akhand (indivisible), nirakaar (formless), that is formless Divinity. Yet, shivling has a form. Why is that?
The science of Vedas is very intense, it cannot be understood by words and meanings, experience is a must. Shiv is beyond birth and yet He has a form. And this form was brought into Creation by Adi Shakti.
People through times have wondered whether the hen came first or the egg. Well, the hen came first. Adi Shakti manifested itself and then created gravity around it. How? Through a whirling motion, through rotation or ‘pradikshina’. Rotation is the basis of gravity, of magnetism, of the energy of prana which is in every microcosm of Creation. When something rotates, it creates a pull or magnetism around it. When you do pradikshina of a Shakti, you are able to relate to it very easily. This practice is used in advanced sadhnas of Sanatan Kriya.
When Adi Shakti created the force, yoni manifested. And shivling manifested in that yoni. If you look at shivling, it is an oval. It is continuous, with no beginning or end. If you focus on it, you will get an experience of the countless circles in all directions which create it. A shivling encompasses in it, the entire Brahmand (Creation). When you do sadhna on the shivling, then the entire Creation unfolds in front of you, and you are able to access any energy therein.
Shiv is beyond birth, there was no birth of Shiv. It was already there, but it manifested from the unmanifest with the force of gravity, by the movement of Adi Shakti. Shiv is eternal, but that specific form of Shiv, manifested from the force of Adi Shakti. When you create that gravity within you through mantra sadhna and specific kriyas as prescribed by your Guru, then you too can manifest any energy, depending on the level of your sadhna and intensity of gravity generated by you.
For this, there is a specific kriya in Sanatan Kriya called the pradikshina kriya in which the Guru prescribes certain mantras to the practitioner and by chanting those mantras while doing the pradikshina of a shakti – Surya, Chandra, Dasmahavidyas, etc. – the energy of that shakti comes inside the practitioner and he/she starts becoming like that shakti. However to become shakti, the bondages of maya need to be opened and so it is important to practice the kriya under the sanidhya of a Guru who knows your attractions and guides you as per your individual capacity.