This exercise seems quite simple but I found my breathing quite constricted when using one nostril then the other. Why did it feel as if I was suffocating?
I know this post is old, but I thought I would answer Melissiah''s question.
If you feel constricted by using only one nostril, it is because of one of two reasons: Either you are used to breathing in using your mouth only, so breathing with your nose is substantially harder/you are used to breathing fuller breaths and felt as though you weren''t taking in as much air; or, you have stoppage in your nasal passages. Doing nose-only breathing exercises, and practicing other simpler pranayams before this one can substantially help your capacity. If the reason was the second, doing this for an extended period will completely clear you out, so air should flow smoothly.
Also, sometimes you expect to feel constricted, so your mind convinces yourself that you are. A good remedy for this is to visualize having to inhale a large object like a golf ball. This will help subconsciously expand your nasal passage and allow for more air intake. Mind over matter.
One nostril is always dominent. It changes every few houres. also use a neti pot daily, you can use salt and baking soda. or the solution that comes with it.