Normally, with the attention left unchecked in a semi-conscious state (habitual state of unconscious reaction) energy goes all over the place following reaction to constant and created stimuli. Tapas is the force or energy which drives the movements of these experiences, reactions, and/or actions. For example, Tapas is the heat which causes you to seek shelter from the sun, the feeling of hunger which seeks to fill the belly with delicious food. It is the feeling caused by the smell which draws one near or pushes one away. Tapas is the friction of impatience building as one waits in line, while imagining all the things that need to be done. Tapasya then is the conscious choice to extract one's identification with, and compulsion to follow these various drives.
This is not some dogmatic, or religious fantasy; Tapasya is pragmatic reorganization of one's source of power from external props to within, and instead of dispersing energy - gathering it as increasing potential under sharpened Conscious direction and focus...
A form of "external" Tapasya or practice would be to stop talking for a period of time. Darshan Baba enjoys doing this practice while still using the mouth to sing, pray, and chant Mantras - but no talking... As you stop talking various things start to happen within the experience of Consciousness which make Tapas and Tapasya very clear as a pragmatic affair.
As you stop talking often you may find that others will find it necessary to talk for you. This may then bring a very concrete example of tapas with a flare of the ego, as the others story may not just match it's (ego) story of what is going on. It (ego) flares up to correct the offender, but you are not talking, so then there is almost this burning feeling (tapas) that wants to hook you into talking. In this tapasya you have taken the commitment not to talk for such a period of time, so you simply sit with this "sticky" or burning feeling that would normally hook you into a whole set of actions with the corresponding effects and continuing reactions to those effects causing yet more effects to "hook" into.
This Tapasya of mouni (not speaking) practice is then offering an opportunity and the self discovery of the methods for actual change in one's behavior leading to a whole new life and possibilities of experience. The story in the mind or voice arises with the feeling of such importance "that i know so much" about what they are talking about, or have something so important to say to contribute. But having chosen not to talk, well, might as well just go to the mantra this mind was repeating...
After days like this, the energy has been redirected, and these burning sensations or itches to engage in the external stimuli, are almost extinguished. Actually that energy or potential to act - which is normally arising and being spent to meet the demands of the reactions to these stimuli - is still present and as potent as when it was experienced as a strong urge to hook into the situation. But now the energy is gathered and focused under a Conscious choice of how it is directed, with greater pragmatic Awareness of and freedom to make a choice.
Imagine if one has been practicing in such ways for years consistently. The mind then becomes completely identified with and established within Inner Reality for its energy and power to cause effects in the "external" world. This is the mind of a Siddha - tapasya has been established as an internal affair. This identity within Reality has been so consistent that no matter what happens in the world of appearances - That Identity within Reality does not move. This is the meaning of this statement:
"He who has seen his true Self, which is innate, unborn, and incomprehensible, does not, if anything desired happens to him, become tainted. Being free from taint, he never performs any action. The man of self-restraint or the ascetic, therefore, is never bound." Dattatreya - Avadhuta Gita Ch. II V.30
This then is a description of "internal" Tapasya, or the essence of what the Sages/Realized Teachers have implied by and wished for there students/us to realize with: Tapasya. The power of Tapas as a hook to engage in external identification or stimuli at this stage has no influence over the Tapasvi (One who has Realized Tapasya). These things of "external" appearances do not distract or hook the Yogi away from a Supremely Centered Internal Reality. As a Siddha then the outer appearances are of little value - where the nectar of Immortality and True Wealth is brimming within. That one then may appear to eat or not to eat but they never eat...
Darshan Baba many years ago read a story - a PERFECT Teaching - which came as a personal story relayed by Roger Montgomery the Author of "20 Count: Secret Mathematical System of the Aztec/Maya" in his book. The Author of "20 Count" -Roger - was at a function with many "important" (famous well recognized Masters/Teachers) . Recognizing a Zen Master who was very famous as being highly realized, he approached and respectfully asked for a Teaching. The master then gathered on a plate many of the amazingly delightful looking items from the catered setup, arranging the plate beautifully and delectable looking. The master then presented the plate - and a Perfect Teaching as he said "Don't eat" and then walked away giggling like an enlightened buddha child... Looking at the beautiful plate of food beyond temptation he of course ate the food, puzzling over the meaning of this teaching.
Darshan Baba contemplated this teaching for many years during periods of Sadhana and Tapasya. Look at Dattatreya's statement... The Siddha regardless of whether they may appear to be deep in enjoyment of the plate of food, or rigidly avoiding all "pleasure" - certainly does not eat. That Free One knows that there is really only one Actor and Enjoyer - and all of this Infinity with all angles, opposites, and diversities is the Magic of, and Belongs Solely to that One!
How Amazing! The Miracle is Here Now!
How Amazing! This Infinite Splendor hides right under everyone's noses! The very Essence of Being, and the Simple Truth - Reality. You can search and wander the worlds, but if you don't look right here - right now, right at your very own Being - discovering what you have Here Within - then where will you Realize the Self...?