My teachers often refer to a model of energy which describes a fire burning inside us, it is the combustion sub-system of our energy system (there are other sub-systems that play other roles). It is said to be located in the abdominal (apana-stana) area – it feeds on air that comes from the chest area (prana-stana) and burns ashes that are located in the lower-abdominal area.
Fire governs digestion and elimination. Like a combustion engine, it runs on a mixture of air and fuel. Air is supplied by the breath. Nutrition plays a key role in supplying fuel. The quality and quantity of the fuel can affect the fire. When it burns well food is digested effectively and efficiently, and wastes are eliminated in a timely and regular fashion. Otherwise – it can cause the system to overheat and become acidic – causing ineffective digestion (heartburn) and elimination (constipation/diarrhea).
The ideas of digestion and elimination also reflect interestingly on qualities of the mind. Digestion can be viewed as a quality of perception. When the fire burns healthy there is clear perception, when it burns low things can go unnoticed, when it burns high we misapprehend – we imagine things that are not there (a quality of over doing). Elimination I feel warrants a deeper exploration – but for now I would align it with purification of perception. For example, when memories of the past and expectations of the future are purified – there remains a clear sense of that which is present.
Yoga practices affect the fire, established practitioners can use yoga techniques to manipulate the fire. Breathing, as both a dedicated practice such as pranayama, and in conjunction with asana, can have a drastic and immediate effect on the fire. Breathing techniques can and do affect the fire in seconds or minutes. Intake and digestion of food usually takes at least a few hours to transform and affect the quality of fuel (and indeed you will find that some intense yoga practices require a careful diet – often one that supplies the body with nutrition which can be easily and quickly transformed into fuels). Both fuel and air have short & long term accumulative effects on the system.