http://www.rochester.edu/College/REL/rel249/siva/shiva_dance.html

Myth has it that in the forests of Taragam,
   there lived many heretical Rsis, who Siva went to confute, and Vishnu
   dressed as a woman went with him. The Rsis fought amongst themselves, and
   then turned on Siva. The Rsis produced a tiger, a serpent, and a monstrous
   dwarf, and sent them one by one to destroy Siva. Siva killed the tiger and
   wore its skin. He subdued the serpent and wrapped it around his neck.
   Finally, he trampled the dwarf (Muyalaka) under his foot. After this, the
   Lord of Dance continues his dance on the dwarf, in the Tillai or center of
   the universe. Upon examination of the dancing Siva, the first thing to
   notice is that the Lord is dancing on the dwarf.


---------

Nataraja Siva

In this form Lord Siva is depicted in his famous Ananda Tandava
dancing pose, with one leg in the air and he has four arms. Nata
means ÒdanceÓ and raja means Òking.Ó Thus he is known as
Òking of the dance.Ó Lord Siva is depicted dancing on a dwarf
named Apasmara, who is said to represent ignorance or false
ego, which makes souls forget their true identity. Nataraja
destroys false ego. 


------------

 Lord Siva is depicted dancing on a dwarf
named Apasmara, who is said to represent ignorance or false
ego, which makes souls forget their true identity. Nataraja
destroys false ego.