Deliberately choosing one nostril to breath through
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3681046/
Assessment of the Effects of Pranayama/Alternate Nostril Breathing on the Parasympathetic Nervous System in Young Adults
Result and Conclusion: This study showed that the parasympathetic tone was enhanced appreciably in the participants. The observations of this study suggest that the yogic exercise of A.N.B. influences the parasympathetic nervous system significantly.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3978938/
Slow Yogic Breathing Through Right and Left Nostril Influences Sympathovagal Balance, Heart Rate Variability, and Cardiovascular Risks in Young Adults
Short-term practice of LNB improves vagal tone, increases HRV, and promotes cardiovascular health of medical students. Practice of RNB increases sympathetic tone and could jeopardize cardiovascular health.

There seems to be some evidence that this works.