A yoga danda is a wooden crutch used as an aid to pranayama and meditation.
It is used to alter the predominant swara of the user; usually to create a balance between Ida nadi (mental energy) and Pingala nadi (physical energy), allowing Sushumna nadi (spiritual energy) to flow.
Swara yoga states that during the day we switch between Ida and Pingala dominance every 60-90 minutes. For a few minutes during the transition Sushumna flows. When Sushumna flows there is a balance in the flow of air in the nostrils, a balance in the pranas, a balance in the autonomic nervous system, we experience a state of equilibrium. When this occurs it becomes much easier to practice pranayama and to develop dharana (effortless prolonged concentration) and dhyana (meditation).
In medical terminology, swara yoga is often called 'forced unilateral right or left nostril breathing'.