Accessory attachment to use when you’re installing pre-grooved decking. The attachment mounts to the back of the tool and registers the level of the fastener precisely off the joist surface rather than the top of the decking. By adjusting the attachment up or down, you can fire fasteners with the spur at the precise height off the joists to match the groove height on the deck boards. You’ll need this when installing dense composite decking and hardwoods to save the difficulty driving the oncoming decking onto the fastener spurs. Some builders leave the accessory attachment on my HIDfast tool and don’t find it interferes with standard operation.