The potion requires a mortal anchor in order to keep the user tethered to the physical plane.
In Careful What You Witch For, while trying to decipher the Book of Elders to regain the Charmed Ones' powers, Mel Vera and Harry Greenwood found a symbol in the book that referred to The Sentinel, a being that is the keeper of all wisdom and knowledge that lived on the Astral Plane. Mel also discovered a symbol with the letters 'O', 'B', and 'E', meaning 'Out of Body Experience', and deduced the page must be about astral projection, referring to the ingredients to make the tea.
While trying to find the remaining ingredients, Harry and Mel went to Spellbound Botanica, where Harry explained to Mel the neccessity of a mortal anchor to prevent their spirits from becoming trapped in the Astral Plane, before reasoning that Katrina would be able to. Mel was initially against the idea, believing Katrina to be a charlitan, but relented when Harry expressed the dire need of an anchor, and that Katrina would most likely be the only person who would meditate with them.
Mel, Harry, and Katrina used the tea a short while later, in a communal area in SafeSpace. The tea took a while to kick in, but allowed Mel and Harry to astral project into the Astral Plane, where they met the Sentinal.
- This potion utilizes astral projection. In the original series, astral projection was a power advancement of Prue Halliwell's during the second season.