Siva means Auspiciousness. Siva worship in the South preceded the worship of Siva in Vedas. Details of Siva , in comparison with Siva in North India, is quite exhaustive in the South.The Avatars Siva are detailed as 64 in the South in ancient Tamil literature. The concept of Self Realisation, God, in the Saiva Siddhandha is the same as in Vedic Thought. The principles explained in Saiva Siddhandha are more detailed , 64 , though the Advaitic Non Dualism is the core. Nirguna Upasna is detailed as also Saguna Aradhana. Saguna Aradhana is centred around Siva. The basic principles are Pasu, Pathi and Paasam. This corresponds to Jivatma( Pasu) Paramatma ( Brahman, Pathi) and Maya(Paasam)
One of the earliest and authentic works on Saiva Siddhandha is Thirumandhiram by Thirumoolar.Here one can get a detailed account of Saiva Siddhandha and also the essence of Yoga.
Siva, as in Vedic Thought is expressed as Sabda Brahman in the Form of Five Aksharas ந ம சி வா ய. These Five letters are the same as Na Ma Si VA ya in Sanskrit.The pronounciation is the same with the same time taken , Matra.Siva ,as in Vedic Thought, is portrayed in Sthoola Rupa, in Gross , as Saguna with Five faces.
As Nirguna Siva is expressed as Pathi and He manifests in Human Body too.Siva remains as Sound, Nadha Brahma, Pranava As Brahman. This is Subtle.This is experienced by chanting of Pranava OM.Si VA ya can also be chanted but it’s Chanting is different from chanting OM, though the results are the Same,Siva.
This Sookshma Siva Manifests in Human Body when Om is chanted. OM, ஓம், Tamil ancient texts slightly differ in explaining OM, in that they say that when one pronounces A, அ,and உ , U mouth remains open and when you close the mouth ம்,M sound rises. Note that A and U are a part of OM in Vedic Thought too. There the explanation is that A U and M constitute Pranava,OM and that A rises from the mouth U from Throat and M from the pit of the stomach. These are the only places from where sounds originates in Human body.This is the Primordial sound and the basis of all spoken words.
In Tamil Thoughts, Thirumandhiram By Thirumoolar,Saiva Siddhandha, A அ represents Siva and U, உ represents Sakthi.
When these two Aksharas( letters) are intoned,OM is completed and Siva and Sakthi together rise as Sadhasiva in the Subtle Level in Human Experience. And This Experience is Athma Linga.Itcis also expressed as Siva is expressed as Sakthi and Siva in Humans as U and A respectively உ அ.
One visuvalises this in the Gross Form as Siva with Five Faces.
அகார முதலாய் அனைத்துமாய் நிற்கும்,
உகார முதலாய் உயிர்ப்பெய்து நிற்கும்,
அகார உகாரம் இரண்டும் அறியில்,
அகாரம் உகாரம் இலிங்கம் அதாமே” Thirumandhiram 1753