Yoga Rahasya Chapter One verse 33
abhyāsa samaye nityaṃ recapūraka kumbhakān |
yathā śakti prakuḥ vīta rītya ujyāyvā vilambataḥ ||
During the practice of Āsana one must constantly engage
in regulating the exhale, inhale and retention.
Ujjāyī is to be smooth and slow,
according to the strength or capability of the student.