Years of intense combat and training under Shas’nel E’taro Lynu’nan have honed this veteran unit to the pinnacle of Tau doctrine in ranged supremacy and the fire caste martial arts.
When this model makes a wound roll of 5+ that shot adds 1 to its AP characteristic - for example, AP -1 becomes AP -2. In addition, add 1 to the attack characteristic of this model.
It makes Strike Team lists significantly more effective if they land their hits and wound well. Combined with Light Rail Rifles you essentially get a Pulse Rifle with potential for -3AP, but it’s very costly to do so. Since they’re not available with other Sept Tenets and unaffected by Pulse Accelerator Drones you’re also stuck with the 30” max range.
The +1A is mostly fluff. Combined with some of the other rules it’s potentially useful, but it’s not turning your S3 Fire Warriors into Space Marines or anything of that nature.