Without spoiling too much, we can tell you that wielding the Light is a matter of having willpower or faith in one's own ability to do it. That's why there are evil paladins (for example, the Scarlet Crusade and Arthas before he took up Frostmourne). For the undead (and Forsaken), this requires such a great deal of willpower that it is exceedingly rare, especially since it is self-destructive. When undead channel the Light, it feels (to them) as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: sure, the wound is healed, but the healing effect is cripplingly painful. Thus, Forsaken priests are beings of unwavering willpower; Forsaken (and death knight) tanks suffer nobly when they have priest and paladin healers in the group; and Sir Zeliek REALLY hates himself.
"There is no good. No evil. No Light. There is only POWER!"
These were the words Benedictus used when he fought Thrall.
To Benedictus Light is only power and with this veiwpoint he surpassed even Velen(point of fact: his Waves of Virtue and his Purifying Blasts were all much larger then anything Velen used)
It seems to me that the best way to draw on the Light is to see it as "only POWER!"
It seems Light is just sheer willpower in the end it seems....