Prána, of a generic kind, can be divided into five more specific types: prána, apána, udána, samána and vyána. These can each be further divided into various subpránas. Prána is visible. On any sunny day, execute pránáyáma and fix your eyes on the blue of the sky. Wait. When your vision adjusts itself, you will begin to see myriads of incredibly dynamic and brilliant minute points that glimmer, making their rapid circular and sinuous movements evident. When executing your respiratory exercises, mentalize that image, that you are absorbing that energy.
(Master DeRose, 2007, Swasthya Yôga Shastra, S. Paulo: Uni-Yoga Nobel, p. 227)
*The accentuation of the non-translated Sanskrit words respects the original pronunciation of the language.