Stripped of mythology, tapas is the deliberate acceptance of chosen difficulty, and the tradition's catalogue of its forms is practical: fasting and restraint in food; vigil and early rising; silence held for set periods; endurance of heat, cold and discomfort without complaint; the steady keeping of a practice on the days one does not feel like keeping it.
The Bhagavad Gita, systematizing, divides tapas three ways: of the body, which is service, cleanliness, uprightness and nonviolence; of speech, which is words that are true, kind and useful, and the practice of recitation; of mind, which is serenity, gentleness, silence and self-command. The same passage adds the tradition's essential warning: austerity performed with ostentation, or to extort recognition, or in self-torture, is tapas of darkness, worse than none. The point of the heat is refinement, not spectacle and not punishment. Patanjali's placement of tapas within kriya yoga, alongside study and surrender, makes the calibration explicit: austerity is flanked by intelligence and humility, fire under instruments, not fire loose in the building.
Modern practitioners meet tapas most honestly not in dramatic feats but in the unphotogenic middle of any long discipline, the fortieth day of the practice, the point where novelty is gone and the only fuel left is the vow itself. That, the tradition says, is where the actual burning happens, and where the impurities Patanjali promises to destroy are in fact consumed.