It is a process invented by Creepy Hollows where the spirit or spell is bound to both a vessel & the Keeper.  This gives the Keeper the freedom of having the spirit with them without having to have the vessel.  It allows them to call the spirit to them when they desire as well as sending the spirit back to the vessel if needed.  In the case of spells it allows the Keeper the ability to call upon the power of the spell whether they have the vessel with them or not.