A skilled helmsman holds the tiller steady against shifting currents. In a similar way, certain seekers master the flow of breath. They offer the incoming breath into the outgoing, or the outgoing into the incoming. Some restrain both currents entirely, holding the breath at a still point. This is pranayama — and it too is a form of yajna.
Pranayama is more than a breathing exercise. Tradition regards it as a way of channeling inner energy and stilling the mind. The body's subtlest movements are harnessed as instruments of spiritual practice.