Yes Sonia and Agent green glass -- both of u get virtual `good-student medals' for getting the answer right:) Sorry u other guys, though exhalation is crucial, the actual act, according to classic texts as well as yoga gurus, lies in holding the breath. It is possible that is so because in retention that life force (prana) expands...
But I thank u others for attempting an answer. That excites me, convincing me that I am not writing here in vain:)
So, the answer is indeed RETENTION.
I know all these yoga schools which teach anulom vilom without the ratio in place -- what sort of convenient yoga they teach? Maybe those who see yoga teaching as a career option, through commercial spectacles, such teachers make do in class (some convenient make-do behavior -- tell the students to do surya namaskar on their own. Request them to go home and do the yoga nidra -- some power -- ha ha -- yoga classes actually do that!!-- , no sequence or structure in place; Lot of talk about yoga philosophy so the student practices less:)
In pranayama such convenient instructor behavior becomes more apparent: because to merely say inhale-retain-exhale is easy. Some instructors also bypass pranayama because students will say that is boring and instructors are afraid of holding on to students' attention. However, the integrity of your teaching cannot be bulldozed by that sort of student demand. Also, if u believe and practice pranayama as the most powerful part of yoga, u can get that across. Pranayama is really the most exciting part of yoga... The crossroads between meditation and asana practice. Between the animal and divine in you:)
What is the actual healthy ratio for anulom vilom for inhalation-retention-exhalation respectively?