What can I gain from alternative nostril breathing?

If ur normal, nothin. Breathing into one nostril while holding closed the other nostril will gain you nothing if the anatomy of your nose is normal theoretically if you have a deviated septum or a "hole" in your septum (say from use of cocaine) holding one nostril closed while inhaling air could give you better oxygenation, though I doubt you would want to walk around holding a nostril all day. Have a dr, examine you!