How does 'Effigy of Combustion' work on zombies?

"When a zombie dies you have a chance to cast infernal shade on an enemy near the zombie.

infernal shade cast this way costs less mana"

I have Summon Volatile Zombie as a main skill, but not infernal shade. I’ve been casting volatile zombies for ~20 minutes, and have yet to have an infernal shades casted. I understood this as the skill would be casted without me having to do it, is this right? How does this work?