Sure you can. I do it in every OW class I teach.
It is in preparation for the horizontal CESA, a required skill for the class. It is an artificial skill I have argued against for a very long time, to no avail. Students are supposed to swim 30 feet in a pool, exhaling the whole way, while swimming at a "normal" pace. An overly anal reading of the PADI ascent material defines a "normal" ascent rate at 60 FPM, meaning that a student who performs this skill under the supervision of an instructor who insists on 60 FPM must exhale for 30 seconds. I have known instructors who do insist on this rate, and their students are able to do it. (As I said earlier, in my IE, I was clearly told that students could go faster then 60 FPM during a CESA.)
Although I do not require students to go at that rate, I do go at about that rate when I demo. Before I do that, I have them practice it while standing in the shallow end of the pool. I do it with them, my eyes glued to my watch while I exhale, indicating every ten seconds that pass. Most of them do not hit 30 seconds in this exercise--its benefit is it teaches then not to blast out all their air in the first 5 seconds of the exercise.
They can choose one of three techniques I describe for them:
1. The "ahhh" sound, with that "ahhh" barely pronounced, the air barely escaping.
2. The "Zzzzzz" sound, again barely pronounced.
3. The "King tut" technique (the one that works best for them usually), in which the student says "Tut tut tut tut tut tut" while exhaling.
Again, I emphasize that this is an artificial process designed to overcome the fact that they are required to do the horizontal CESA with no air expansion. Most importantly, I would never, ever, ever advocate this as the way to breathe normally while diving.
