An attachment for a fluid-operated percussion drill tool, the drill tool having a backhead assembly for attachment to a drill rod. The attachment comprises a shaft attachable to the backhead assembly, wherein the shaft comprises a central bore in fluid communication with a pressure bore of the drill rod. The attachment further comprises a sleeve co-axially slidably mounted on the shaft, the sleeve comprising an outer shoulder at a rear end thereof, and at least one rearwardly-directed exhaust passage. Pressurised air supplied from the central bore of the shaft to urge the sleeve rearwards into a closed position in which air is prevented from exhausting through the exhaust passage. When a restriction is encountered at the shoulder, the sleeve moves forward to an open position in which air is permitted to exhaust through the exhaust passage adjacent to the restriction.