To understand the brilliance of Charmed 6x01, one must recall the explosive finale of Season 5. The sisters had just defeated the Titans, a feat that required them to temporarily become Goddesses. In the chaos, Leo had become an Elder, a promotion that necessitated his permanent departure from the mortal plane to "Up There." This left Piper heartbroken and the Power of Three potentially fractured by distance and duty.
Under a "memory-wiping" spell gone wrong, Piper is unnervingly happy and detached from the pain of losing Leo, which causes her powers to manifest in dangerous, uncontrolled ways. Phoebe's New Power: Phoebe develops the power of charmed 6x01