Practical Implications of seemingly esoteric yogic concepts like Agni and Prana
I wrote an article recently titled "Your most valuable possessions for Yoga -Agni and Prana", where I described two of the most fundamental concepts around which entire Hatha Yoga practices (for example Ashtanga Vinyasa yoga, Vinyasa krama yoga and other pranayama techniques) are based on. Agni refers to the body's fire and Prana to the life force, which includes the breath. The question that will naturally arise in the minds of some people is "Ok. These seem like nice academic concepts of yoga but of what practical use are these and how will the knowledge of these help me make more informed decisions about my diet and lifestyle. Most importantly, how do I get a feel for these in the same way I feel my heart rate". Those who are teachers of yoga may have an additional question "How can I teach these to my students in simple terms that they understand".
2) The most noticeable effect of regulation of the Prana
Every classic Hatha Yoga text talks about the agni and the prana. One such text is 'Yoga Yajnavalkya' which takes place as a question and answer session between Sage Yajnavalkya and his wife Gargi. Mr. A. G. Mohan, in his, commentary on this text in page 10-12 (page 15-17 of the pdf) describes how the entire goal of yoga is to regulate the flow of prana to flow along a certain direction and not scatter it. He says that the typical height of a person as measured by the girth of the fingers is 96 Angulas (Angula means finger). However, the Prana of a typical person is so diffuse that it extends to 108 angulas. For a yogi, the Prana is regulated and therefore extends less than 96 angulas. I had explained this regulation in the article titled "Bandhas Demystified (without cosmic consciousness)" where I explained that the purpose of the bandhas is to channel the flow of prana in a certain direction. In Marichiasana-D, shown below, the Prana is channeled along the spine. You would get the feel of a rising energy along the spine. When the Prana is not scattered, the mind settles down. This is the first effect you see in asana practice of the regulation of prana.
3) Feeling and Safeguarding your Agni
How Agni is felt in your body depends on your dosha and Vikriti (imbalanced state). For example, people with a Vata imbalance can face vishama agni or intermittent fire as descibed here. Pitta imbalance causes excess fire and Kapha imbalance causes a dull fire. Asana practice strengthens this Agni. When one practices asanas with the Ujjayi breath and the bandhas, it will feel like a burner starting in the stomach with increased alertness and a healthy appetite. As mentioned in the previous article and also by Mr. Mohan in page 12 of his text (page 17 of the pdf), in Ayurveda, the root cause of most diseases is the inability of the agni to burn down the food resulting in deposition of toxic substances in the body. Therefore the agni has to be protected with fervor. As we saw, excess eating douses this fire. The rule of thumb for yoga practitioners is "Eat just as much so you can still engage your Moola Bandha".
Padmasana where traditionally all the three bandhas (Moola, Uddiana and Jalandara) are engagaed
4) The ancient wise adage
There is a traditional adage that when one is sitting still for long periods of time (for example in an airplane), it is important to eat less. This is because the body's fire is not fanned by the demands of physical activity and is therefore naturally impaired. On top of it, if one, out of bordedom puts more and more food in the body, it causes the fire to be nearly extinguished resulting in the accumulation of toxins. This seems almost like a cliche, but unfortunately, in intercontinental flights, out of boredom, many resort to eating. Eating less on the other hand keeps one relatively fresh and light even after a long flight. This is perhaps one of the most directly relevant implication based on the concept of Agni for our health.