From DT Online
Snarling is a process used to hammer out tall Raised forms or narrow-necked objects from the inside by striking the neck of a Snarling Iron held in a vice causing it to vibrate against the inside of the vessel.
A Snarling Iron is gripped firmly in a vice and the object slid over to position the job on the head. The neck is struck sharply with a hammer causing it to rebound against the inside of the object. After each blow the job is rotated to its next position and the iron struck again until the desired shape is achieved.
Snarling can be used to correct dents or imperfections in existing products or to create raised designs on them. The rounded ball end may be substituted with other shapes to vary the embossed designs.