This question came: Why is tapas sometimes translated as austerity? Is austerity a precondition for zeal, or vice-versa?
Firstly, we need to remember that the meaning of words is always relative to the context in which they are used. In this light, tapas actually has several meanings depending on the usage. The dictionary defines tapas as warmth, heat, pain, suffering, religious austerity, self-mortification, penance, severe meditation, special observance, etc. Of these, what is the most literal meaning of tapas, and what does tapas mean in yoga? Literally, tapas is ardor connoting a combined sense of enduring effort along with a burning heat. The other dictionary meanings are various things that have at their core this literal meaning: effort & burning.
In India, the performance of penance or severe austerities is done by extremists for various reasons. Perhaps to catch the attention of a god or demigod and to then be granted a boon. Perhaps to gain an esoteric power such as clairvoyance or to gain undefeatable strength or even to gain wealth or fame. These are all worldly reasons though, and hence, although they are types of tapas, these are not yoga. Yet this is the image that commonly comes to mind by most people (at least in India) when hearing the word tapas.
What we need to know is what is tapas from a yogic perspective. Yoga teaches that the practice of resisting our own self-created compulsions, the force of our own self-destructive habits is tapas. This type of tapas is the process of pausing and mindfully being aware of what we are about to do & why. So rather than thinking that we should be angry in order to protect our interests against so & so, we take a moment to reflect on the true causes of happiness & suffering. Or rather than compulsively or mindlessly eating the entire ____, we pause a moment to think about the consequences to our immediate & long term health. We know that negative actions generate negative results, and that only good & selfless actions can generate true joy, yet we still allow ourselves to be swayed by faulty logic: The compulsion persuades us to believe the action will be fun & will lead to more happiness, but experience proves otherwise. The act of resisting these compulsions requires lots of effort, and this is where we benefit greatly from wisdom: The knowledge that the impulses actually lead to a lose of vitality & joy and hence there is nothing noble or worthy in the indulgences of such things. Our initial resistance is just like a brake that heats from the friction. We experience a mental or emotional friction within ourselves and a purifying heat is generated. This heat is also experienced as a type of pain. However, we should know that the pain of resisting such unwise acts is very short lived - like the sting from an ant bite. Whereas the pain from indulging in compulsive habits lingers for hours, sometimes days or even longer. So resisting unwise impulses (yogic tapas) follows the literal meaning: an enduring effort that produces a burning heat.
Fever means that we have so much interest in something that we pursue it with seemingly unending energy & enthusiasm. This goes hand-in-hand with tapas. If we don't have enough energy, then we won't have the vigilance to exercise the discipline of tapas. So fever or vīrya is certainly an asset to our practice. The vitality of our energy comes from the level of our conviction (śraddha). If our conviction is weak, then our pursuit will be weak; if strong, then our pursuit too will be vigorous. Simply imagine anything that we do that we strongly believe in doing. Don't we then pursue that with more intensity than we pursue things which we don't feel very strongly about? So we approach our yoga practice according to how we prioritize it in our life. Likewise, we will pursue the examination of our motives (yogic tapas) according to how valuable this seems to us.
With this in mind, in order to inspire a deeper interest in yoga philosophy, we need to understand the laws of cause & affect to see how our health, our happiness & our suffering truly arise. This requires study. We must study to understand the value of what we have in front of us. Otherwise we will be like someone in possession of diamonds, but who views them merely as common pieces of glass. Improving the quality of our lives is a completely inside job - one that we must get inspired to do within ourselves. No one can do it for us, but we can support one another in the arduous journey of self-transformation called yoga.