in Asana Practice
Some people are naturally flexible. Others have to work hard to achieve flexibility. In the world of yoga there are a lot of stereotypes regarding flexibility, for example, that women are more flexible than men and that younger people are more flexible than older people. While there may be some truth in all these, what is often ignored is that in order to twist and bend in yoga poses, innate flexibility while certainly helpful, is not a limiting factor. There is another type of temporary flexibility, that is gained during asana practice while on the mat, using certain yogic techniques like the Ujjayi breathing and the bandhas (energy locks in the body) and the maintenance of a clean intestine. I call this type of flexibility "Malleability" because just as heat is used to forge metals into a desired shape, these yogic practices generate internal heat (tapas) giving us a sense of lightness and the ability to flex our bodies well beyond what we can normally do.
B.K.S. Iyengar in Marichiasana-D, a deep spinal twist in which the heel of one foot presses on the abdomen stimulating the digestive juices and giving the feeling of igniting the fire (Agni) in one's stomach
The Heat of Tapas
All of the four popular texts on Hatha Yoga - Hatha Yoga pradipika, Gheranda Samhita, Shiva samhita and the Yoga Rahasya- talk extensively on the tapas or the heat generated by the Hatha yoga practice to remove diseases in the body and cure the body of ailments.
(Gheranda Samhita 1.8)
The Ujjayi breath
The first of the two components that are commonly used in asana practice to produce the internal heat is the Ujjayi breath. To emphasize its importance, Gheranda Samhita (5.67) says that "to destroy decrepitude and death one should practice Ujjayi".
The second component used to produce heat is the Mulabandha (the root lock). It is Not just all the hatha yogic texts that place a great deal of emphasis on the Mulabandha but AdiShankara himself emphasized the importance of this energy lock to raise the kundalini.
Prana and Apana give great success in Yoga when they are united by Mulabandha. (Hatha Yoga Pradipika 3.64)
The gastric flames shoot high when the apana ascends and comes in contact with the sphere of fire (Hatha Yoga Pradipika 3.66).
Then the gastric fire and the apana join the naturally hot prana, stoking the body's fire (Hatha Yoga Pradipika 3.66).
The sleeping Kundalini is awakened by this heat and it hisses and straightens like a snake struck by a stick (Hatha Yoga Pradipika 3.68). SwatmaRama, the author of Hatha Yoga Pradipika, says, that for the reasons mentioned above, one must practice Mulabandha everyday.
I am innately not very flexible. Further, many years of running and also a prosthetic hip added to my inflexibility. However, what I am able to do in my Ashtanga yoga practice is mainly due to the use of the aforementioned techniques. The feeling I get after the use of the Ujjayi breath and the Mulabandha is akin to a fire in the stomach which gives a sense of lightness to the body and a sense of punch that helps me do poses that I normally would be scared of or reluctant to do. There are of course, good days and not so good days, depending on sleep, what I ate the day before etc., but without these techniques it would not be possible for me to do 75% of the poses that I do.
In conclusion, yogic texts talk about the use of tapas in asana practice through the Ujjayi breath and the use of the Bandhas to go deeper into one's body and tap into the prana or the life force. As a by-product of the heat generated by this tapas, one also attains prowess in the asanas that one does. It is for this reason that in traditional yoga systems like the Ashtanga system, emphasis is laid on the continuous use of the Ujjayi breath and the bandhas. Personally, I have felt my body becoming stiffer and less flexible when there is a lapse in the use of the Ujjayi breath and the bandhas either due to distraction or due to the need to talk to a teacher. It, then, requires additional effort to get back into this state.