While stage one of this meditation practice helps to develop more calm (by emphasizing the qualities of the out breath), and stage two helps to develop more energy and awareness (by emphasizing the qualities of the in breath), the third stage emphasizes both the in breath and the out breath equally. This helps us to blend the calm relaxation of the first stage with the energized awareness of the second stage.
In our meditation practice we are ideally developing a sense of energetic calm awareness, or a calmly energized awareness. While doing stage three you can be aware of the constant oscillation between the calming out breath and the energizing in breath, and allow the qualities of the out breath and of the in breath to permeate each other.
Modifying an analogy the Buddha himself used, you can think about making dough. When you’re making dough, what you’re doing is taking two contrasting substances – a wet one and a dry one – and combining them together in a perfectly balanced blend.
If you have too much water, then you’ll have a sticky mess, while if you have too much flour, you’ll have a dry, cracked ball.
Get the proportions just right, and you’ll have dough that is perfectly pliable and workable. (The Buddha’s analogy involved a “bathman or bathman’s apprentice) blending soap powder and water — presumably the Buddha was more familiar from his earlier life with bathing in luxury spas than he was with baking).
Just as the right balance of flour and water (or soap-powder and water) produces a pliable mixture that can be used appropriately, this stage of the mindfulness of breathing meditation practice helps us to develop pliability of mind; to get our minds into a calm and energetic state where we can work to develop a much greater degree of concentration.