Prāṇāyāma, as with Āsana and Dhyānam, was taught according
to the principles of Cikitsā, Rakṣaṇa and Śikṣaṇa Krama.
Thus we have breathing practices ranging from Cikitsā using simple ratio to settle an irregular breath,
to Rakṣaṇa with competence and fluidity with various basic techniques and mild ratios,
to Śikṣaṇa and mastery of all techniques, and ratios and especially,
the Kumbhaka with long holds both after the inhale and the exhale.
The Vinyāsa Krama or steps in the evolution of practice are measured
by our practice abilities and consistency and potential within our life situation.
The longer term measure of our Prāṇāyāma potential is determined by
our skilful efforts with all four components of the breath in Āsana.
For example can we maintain 126.96.36.199. in Parśva Uttānāsana or 188.8.131.52 in Mahā Mudrā?
These days though, it seems that there is not much place for or interest in the use of Kumbhaka
and breathing practices, if used at all, appear to be mainly Cikitsā or about recovery,
or at best Rakṣaṇa or constitutional, rather than Śikṣaṇa and developmental.