I may sound weird here, but bare with me. **edit** bear with me (lol, thanks Borate).
I've started designing a dryland swimming improvement concept, as you may know already (http://blog.swimtrainingday.com/en/classes/dryswim
) which is followed by a pool technique session.
Every week we approach a new topic, and work on that topic only. 2 weeks ago, we worked on Breathing, bilaterally, and low profile. One of the important keys to this I've found over the years is the ability to Popeye your mouth. Mind you, it could be me (as I really do it fairly well), maybe others don't do it, but I truly find it helps a lot to have to ability to seal half of your mouth.
Now, to my big big surprise, I realized that this seems very hard to do for "most" people participating to this project. We spent quite some time in front of a mirror trying to Popeye on every side (that was our funniest session, we really looked like a bunch of monkeys). Some can do it on their good breathing side, but absolutely can not on their bad side. I think there may be a link.
This has to do the easiest ability to develop home in front of a mirror. Just ensure you're not seen.