One begins to experience a heating from within, or an inner fire, through the various yoga practices. It is kept alive through a disciplined commitment to the practice.
II.43 (Ch 2, Sutra 43) kaya indriya siddhih asuddhiksayat tapasah
Self-discipline (tapas) burns away impurities and kindles the sparks of divinity.
The sanskrit word "tap" means "to burn".
We often hear of the purifying effects of yoga which cleanse the physical and energetic pathways of the human system, clearing the way for the enlightened state of being.
Tapas, that inner flame, is kept bright and glowing, neither reduced to a flickering flame or brought to an out of control wildfire, by our disciplined practice. Postures, breath -work, an even state of mind to the ups & downs of life, mantra chanting, devotion to the Divine, are all contributions to igniting and maintaining tapas.
Ultimately this inner fire can help us to burn through our samskaras, or our karmic tendencies which limit us to repeating patterns. Spiritual freedom and mastery of the restless mind, empowering and strengthening your life for optimal results, and health of the body can be obtained through observing tapas, the 3rd Niyama.