Surya: Sun, solar, heating, right nostril
Chandra: moon, lunar, cooling, left nostril
Bhedana: clear, cleanse, free flowing
The solar right side is said to be like the sun: fiery, generating heat and light, active, and masculine. The lunar left side is said to be like the moon: cooling, receptive, dark, moist, and feminine. Through alternative nostril breathing and kumbhaka, and using this pranayama, you aim to balance your solar and lunar energies.
Like bhastrika (third pranayama), the fourth pranayama offers the possibility of a longer retention because the retention is performed only after inhalation (antara kumbhaka) and not after exhalation.