An underslung loop is a loop whose fly leg unrolls below its rod leg. The current definitions list this shape as an under-running loop; “underslung” is the name the question uses for it.

The point of the definition is position, not fault. A fly leg that passes below the rod leg is the whole test. Below does not by itself mean tailing: a tailing loop is defined by a concave fly leg that dips toward and crosses the rod leg, and a trailing loop crosses the rod leg without a concave fly leg. Use the definition that fits the shape you see.

This is the kind of question where a familiar word invites a wrong answer. “Underslung” sounds like a fault, so candidates reach for a tail. Read the definition and answer with the geometry: the fly leg unrolls below the rod leg.

In a lesson, the useful contrast is between a normal loop and one whose fly leg passes underneath. Have the student watch from the side as the loop unrolls and name where the fly leg sits relative to the rod leg.

Source

From the FFI Fly Casting Definitions (August 2025); terms follow the FFI Fly Casting Definitions (Under-Running Loop).