One of the many meanings given to tapas, the 3rd niyama, is heat. Heat is a quality that implies movement. Tapas is an active principle. Movement is necessary to overcome the imbalance of tamas, the guna of solidity, which at its extreme can manifest as torpor, tiredness and rigidity.
When our lives become out of balance and we find that we are stuck, not moving, we can overcome this either through shauca or tapas. We may find that we need to free up space, clear out the house, clean up our body, in order to be able to move in the first place. This is where shauca comes in. Sometimes though, we have to ignite our inner fire, start up our engines and just get moving. This is the function of tapas. Shauca and tapas can easily go together, tapas providing the heat and movement to clean things up in our life.
There are many forms that tapas can take. Tapas can be inhibitory. For example we may find that we are lethargic due to bad foods or habit patterns that are governing our life. Tapas can take the form of restricting certain things from our life in order to free up the energies that are blocked from the bad habit patterns. If I notice I am tired after drinking every night, I may choose a form of tapas that eliminates alcohol from my life for some time. If I notice that I can't seem to get anything done because my life is a mess around me, I may have to take the time and energy to clean it up (tapas and shauca combined). Tapas can also be a practice which focuses our energy. The later limbs of yoga like asana, pranayama, and mudra can be powerful forms of tapas which help to align our energies and concentrate us.
It is more effective if we choose forms of tapas carefully and work with only one or a few at at time. In this way, if it is used with consistency and moderation, it can build power over time and slowly eliminate the blockages that dam our inner rivers of movement potential. Pick one practice to begin with. It may be abstaining from a bad food. It may be going for a walk at the same time everyday. It may be forcing yourself to write or do that project you have been holding off on. It may be choosing to abstain from certain speech or actions. It may be taking a yoga class or learning a particular pose. It may be learning to slowly hold a pose or meditation for longer and longer. The forms that tapas can take are literally endless and should be adapted with care and discernment according to temperament and ability.
Tapas has the potential, if not used with discernment, to quickly fall
into the trap of asceticism and extremism. Like santosha, tapas should
be used in a balancing way, being careful not to swing the pendulum too
far in the other direction from the imbalanced direction that we were
correcting. If used with careful discernment, tapas can be a powerful tool for bringing the gunas into a state of harmony.
If we are not careful to choose a form of tapas properly, or pick a form of tapas too extreme for our abilities, we may fail. In failing we run the risk of falling back into the tamasic patterning that led us to the tapas in the first place and it may be more difficult to start up the movement again. For example, we may decide that the project we have been holding off on for so long needs to be completed in one day. We try and fail and decide that it "just isn't worth it". Or we attempt to quit all at once eating ice cream, which we have eaten every day for the last 10 years. If we fail, we may decide to just eat more ice cream and feel bad about ourselves. Better to start the project with reasonable consistent goals, or to start out our plan only eating 3 to 5 small spoonfuls of ice cream a day and slowly cutting down, than to push ourselves right to the finish line in one go.
Thus tapas needs to be carefully tempered with discernment. If it is utilized in this way with intelligence it can be a very powerful tool for purification and clearing.
There is an important point about tapas which follows from the laws of physics. If we look at the law of momentum, it tells us that momentum equals mass times velocity. In other words to get mass in motion we have to "give it a push". However once that push is given, it will continue with the same velocity minus the occasional friction that attempts to impede it. Tapas functions in the same way. We will feel the most difficulty in the beginning as we are setting things in motion. Once the motion is set, the pushes required to continue the motion become much easier. To see this in action, start to push a car from neutral on a flat road.
The only thing that slows momentum down are external forces like friction and gravity. Assuming a fairly level ground of movement, this will require occasional pushes to keep the object moving. From this principle we learn that tapas really only bears fruit if it is joined with consistency. It will do no good to have a practice for one day or one week. To really bear its fruit, tapas must be a daily endeavor. This is why it is important to choose one's tapas carefully as we only have energy for so much. Choose a form of tapas that truly works for you, according to your abilities and then once chosen, attempt to stick with it for a set number of days. Traditional tapas periods are 40 or 44 days. If one can stick to that time period with success, then one can decide if one is ready to increase the number of days to a longer period. It may be with certain addictions that one has to really come to terms with a lifelong tapas and makes the decision due to health and other factors that one needs to keep the constant fire lit throughout the remainder of one's days. This is difficult in the beginning but like the law of momentum in physics, the hardest part is always in the beginning. Once the ball is rolling, you only have to overcome occasional friction, and if it is kept consistent, the initial pain of overcoming the weight itself is kept at bay.