In traditional type founding, a punchcutter uses this specialized tool to strike the internal hollows of characters, such as the enclosed bowls of an 'o' or an 'e', into the end of a steel bar. This ensures that the depth and shape of the negative spaces are perfectly uniform across the typeface before the outer shape of the letter is filed away.
Heard on the job
He spent three hours filing the counterpunch for the lowercase 'g' to get the bowl perfectly round.