Tapas - 3rd Niyama
Using the 3rd niyama (self discipline); tapas - is keeping a sustained practice but also corresponds to Pratyahara-sense withdrawal.
Being directed towards the inner core of being and, like the blacksmith’s bellows, it must always continue to heat the heart of the fire of practice, otherwise the alchemical transformation through extreme heat will never take place.
The fire will be burn merrily, but it will not turn lead to gold.
Ref Iyengars Light on Life page 263.