Yoga Sūtra Chapter Two verse 1
tapaḥ svādhyāya-īśvara-praṇidhānāni kriyā-yogaḥ
“The activities of Yoga are self-discipline, self-study and contemplation on the divine.”
“Patañjali has proposed 3 approaches to verify the indications.
Tapas – Process of action
Food, Āsana, Prāṇāyāma.
You will be doing something that you will not be habitually doing.
For example one day no salt, cigarettes, Prāṇāyāma.
Tapas is from the root to create thirst.
It means to deprive.
It will tell us about ourselves.
It will reveal our Saṃskāra and Pariṇāma or changes in ourselves.
From this Tapas we will start to get an indication of our individual nature.
For example active or lazy.
Tapas indicates the the beginning of the Bheda, through the Bhāva.”
– TKV Desikachar France 1983