A line thru a wavy line inverts it. Carcassi gives the wavy line as a mordent, which by then was simply a double grace note using the upper auxiliary. To Bach, it would be the lower auxiliary, and it was only used in descending. In the interim, musicians lost patience with the nonsense of mordents, inverted mordents, Pralltrills, Schnellers etc., all meaning the same thing, depending on whose book you read. A line through a turn, one might safely presume, would do the same thing, although a turn was inverted by standing it on end. (A turn, or gruppetto, is a backward S lying on its side.)
