I understand that the Iyengar perspective is that ujjayi breathing is "coarse" and unrefined.
I think the "yin/cool" Iyengar idea is that we should aim always for the breath to be more and more subtle, more and more refined --with the body viewed as an alchemic vessel, the subtle refined breath will ideally kind of "ooze" out the skin on exhale as a key part of the "distillation" process.
This is in direct opposition to the "hot/yang" Pattabhi Jois approach, which has ujjayi throughout the practice. They are right away firing up tapas, heat, (and real sweat) and melting down, working in progressively from the gross to the subtle in the progression from first to, I think, seventh(?) series. Of course each series takes years to perfect --who the heck ever gets to the seventh series??? I don't know.
I think the Iyengar guys also regard that sweaty Ashtanga tapas as "coarse" as well.
Strict Iyengar certified teachers won't even allow you to do pranayama until you've done asanas for a year or two, refining. Jois takes the more "jump right in" AYP-friendly approach.
It's weird that Jois and Iyengar's schools of yoga are so diametrically opposed (on breathing and everything else), kind of yin and yang in their approaches, but that they actually come out of the same teacher.
Thanks for this discussion, this is interesting. I'm going to try and do a little more research on apana, etc. this evening if I get a chance.
j