I think it's very odd and shows a great lack of imagination that The Charmed Writers wrote it can be taught to those who does not possess Astral Projection naturally. What next how to orb with wiccan orbs.
Or how magical women like The Charmed Ones can use Molecular Combustion when in that time of the month. Or Transmogrification when The Charmed turned into The Blue Moon Beasts. or to become a Telepath.
They should have just tried to extended their powers to its limits.