Traditionally the five "elements" that make up our body-mind: earth, water, fire, air, and ether, are each energetically associated with a fixed point on the linings of our nostrils. So it's possible to influence and transform our body-mind by channeling our breath from these points. These points (two in each nostril) are the "inner nostril" just beside the septum, and the "outer nostril," underneath the nose.
Sit comfortably and attend to your breath as it passes in and out through your nostrils. It's likely you'll feel a difference between the two; your inhale, for example, is touching your right nostril near the septum, but your left nostril below the wing. Watch for a minute or two, then begin to channel (or "narrow") your inhales across your inner nostrils. Continue for a minute or two.
After breathing normally for 30 seconds, begin to channel (or "widen") your exhales beneath s your outer nostrils, "widening" your breath. Again continue for a minute or two, then return to normal breathing for 30 seconds. Finally, combine the inner and outer breaths and breathe slowly for several cycles.