Jjnana caramcan 100, verses 39 and 40

39

The twelve and sixteen totaling twenty eight, 

All these are the deliberate play of the fire;

Inhale and breathe in the ascending 16 petals;

Perform recaka(exhalation) sincerely inside the twelve;

In that fire house is the the beautiful lufe;

One who knows both the great places is a yogin;

O! Dear! Sixteen is the bottom layer of the maiden(crown chakra);

Look tenderly the twelv steps of the feet.

Twelve = heart chakra

Suxteen = throat chakra

The feet in heart chakra, i.e. worship shakti's feet in heart

40

Gently inhaling in the 16,

Without excitement suspend and blow in the twelve,

In the vital body fire has gone;

In the two refined branches current has gone;

Who will tell in public the technique of breathing?

Performing the rare tapas will yield results;

When doing constant practice

Keep the mind in the middle of the two petals(3rd eye chakra)

Fire has gone means desire has ceased in vital body

"Curent has gone in two branches, = ida, pingalai", i.e. prana is now in susumna in middle

So this is a strong breathing, meditation combination pryctice in verse 40. 

All very simlar to heart breath meditation. If sidha technique was predating all the others by thousanda of years then we see here its origins translated from tamil literature and used in buddhism, islam, christianity.