The Opening Spell is a spell capable of amplifying a portal so that people can travel through it. In a manner of speaking, it can turn "a window into a door."
"Elements of the universe. Within and without. Unlock the door, turn world about." - The original incantation used to turn the window portal into a door.
"Elements of the universe, within and without. Space be no hindrance, no barrier, no bar." - The altered incantation used to reopen the portal and bring Andy home.
To amplify the portal into Negative Space, to turn it from a "window" (which only allows communication), to a "door" (where one can travel through).