The idea behind it is that it cuts two or more parallel lines at once, depending on the line count on the graver ( the one shown here cuts six lines at once). So rather than using a tint tool to lay down lines side by side using your eyeball the multiple tool will give perfectly spaced cuts.
A famous practitioner of this esoteric tool was Hans Mueller, an American illustrator from the 1930′s and 40′s. He published a beautiful book called Woodcuts & Wood Engravings ~ How I make Them. I remember my brother chiding me years ago because I found it hard to accept the $60 I had to pay to buy a used volume from an antiquarian. A laughable sum today.
Pictured here is a detail of a wood engraving by him that was done almost entirely with a multipletool graver.
You can click on the image for an larger view.
He was also known for making three and four colour wood engravings using the pinhole registration technique. I’ll write an article about it at some point.