To cast the spell, one must perform the kata and recite aloud "Spectrum of light, cleanse and purify!". It can undo any damage done to anyone, whether from magic or evolution. This spell is so powerful it can be only cast once. However, it would seem that once cast, it would allow the user to perform the effects on a much lesser scale.
In the TV Series
The Spell of Radiant Purification was first used in The Evolution Will Not Be Televised. Upon Raiden being possessed by the Light Mystic, the other four Mystics possess the bodies of Gabriel Wallace, Allison Underhill, Alakshmi Verma, and Lucy so that they can guide them. Upon the spells being fired onto Raiden, he casts the Spell of Radiant Purification which undoes the effects of the Weapon of Mass Evolution. It also undoes the curse that Megaria casted on Master Jaha which makes her younger than she appeared. Due to the spell's effect, Raiden was able to restore Ken Okamoto and Humonculon the Blaster without being able to cast the spell.