Source: https://sanskritdocuments.org/sites/giirvaani/giirvaani/nataraja_symbolism/nataraja_symbolism.html
The left upper hand of the Nataraja idol holds Eternal Fire. He is already surrounded by devastating cosmic fire, then why handling this fire like a torchlight... This handheld fire is the essentail principle of life, with which everything transforms, either biological bodies to ashes, or seawater to raincloud etc. The process called Five-Fire Process, pancha agni vidya stems from this fire alone, which in a rudimentary way is this:
When humans on earth oblate watery substances like curds, ghee, milk, or at least water for gods, gods receive it and in turn oblate it in the celestial fire, called dyu loka agni, mixing with soma, the nectarious principle.
Again when gods oblate that nectarian soma into parjanya agni, fire in raincloud.
It becomes rain, and showers the same in the fire of earth, bhuumi agni, whereby earth yields food.
Then the gods oblate that food in the fire of males, puruSha agni, i.e., they destine males to consume that particular food with nectarian principle in it, which gets transformed as a fruitful seminal fluid in males.
And fianlly the same gods oblate the same fruitful fluid, via their fortunate males, in the fire of females called yoShita agni, whereby that soul which originally oblated liquids to gods takes birth as an offspring of that couple.
Thus the cycle of birth and death becomes continuous.
This eternal fire keeps on burning from birth to death, causes Five-Fire Process, and thus it keeps the Universe on its wheel, as such shiva holds it, rather controls it.
