Details:                   A two-part ritual consisting of a potion to be ingested and a spell to be cast as a spirit.  

Since only a spirit can vanquish another spirit, this potion will stop the heart of the caster and in effect kill them, allowing their spirit to leave their physical body temporarily to cast the spell. They should easily be revived with CPR so long as it is administered within four minutes after their demise.  

Prue drank this potion in order to vanquish the spirit of Jackson Ward.  Andy then revived her with CPR. 

Episode:               S1E20  "The Power of Two"