Something Wicca This Way Goes - S7-E22
Corrected entry: The sisters are preparing for a big showdown with Zankou and vaguely discussing their plan. They don't outright say it (so as not to give it away) but discuss how they wished they "had learned it sooner" but "Prue was very protective of it." They're discussing astral projection which originally was stated as a Prue-specific power and not one that could be taught. The show ignored the original introduction of the power as developing naturally as one of Prue's powers, and was not an ability that could be learned, just so the episode had a way for the sisters to defeat Zankou. (00:30:20)
Correction: In which episode did they say astral projection couldn't be learned? In the series, the sisters aren't the only one to learn to astral project and the series has indicated it is one of the few powers that can be taught. Additionally, others have gained this ability through spells or other means. The monkey injected with Prue's blood could astral project.
Bishop73