Within the practice guidelines for Prāṇāyāma
Length and Subtlety are the fruits of a process,
they are not tools for use within this process.
The tools here are Deśa, Kāla and Sāṃkhya.
By these the breath becomes Dīrgha and Sūkṣma,
in other words the breath becomes Long and Subtle.
– Paul Harvey on Yoga Sūtra Chapter Two verse 50

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