Cuts something open.
Harry used this spell on Cedric's book bag to slow him down so Harry could tell him about the dragons.
Hermione used this spell to cut Ron free of the magical ropes.
Harry used this spell to try to cut Mrs. Cattermole free of the chained chair, but it didn't work.
Harry used this spell to break the ice on the pool containing the sword of Gryffindor in the Forest of Dean.