Yoga demands that you evolve and transform beyond your normal understanding of yourself. Anything in yoga that forces you to confront your limitations with the intention of moving past them can be considered Tapas. Translated from Sansrkit as accepting pain as help for purification, you can think of Tapas as experiences that you endure with a happy attitude in order to strengthen your spiritual resolve. Everything from strenuous physical yoga posture to a vegetarian diet to meditation retreats to sexual continence and much more can be considered some of the austerities modern yoga practitioners can use to train their minds to be bent towards enlightenment. While many avid yoga practitioners take on Tapas with an urgency the practice of austerities should always be done with a Sattvic or peaceful attitude in order to ensure that you will truly benefit on a spiritual level from your hard work.