Googling “vary the line” brings up this patent description: “A bite indicator is described comprising a body (10) to be mounted on a fishing rod support, a rigid arm (24) pivotably mounted relative to the body (10) and releasably connectable at point spaced from the pivot axis to the line of a fishing rod resting on the support, and means (26) for resiliently applying an adjustable torque to the arm (24) to enable an adjustable force to be applied by the arm to the fishing line.” There’s a poem in there somewhere. Or operating instructions for writing one.