Use of the Viper D-Loop Pliers is discussed by Lancaster Archery Supply TechXPert Randy Groff. These pliers are used to tighten a D-loop tied to the string of a compound bow.

The D-loop is tied onto the bowstring at the spot where an arrow should be nocked. The archer would then hook a mechanical release to the loop in order to draw the bowstring.

The Viper pliers lock the D-loop in place on the string, so that it doesn't slide up or down, and so that it won't spin around the string.