This spell conjured a force field. It was used by Willow Rosenberg to impede the attacks of the Knights of Byzantium against the Scooby Gang. The force field created by the spell was incredibly strong, it was impervious to most forms of physical attacks. Several armed knights and even the strength of a slayer could not bring it down.
Although powerful, it could be temporarily broken by beings with immense strength, such as Glorificus.
The clerics of the Knights of Byzantium claimed that they could tear down the barrier created by the spell.