This is how I paint what I think is a gyroscope on all my T'au models. You'll find this element on models of different sizes and races across the T'au army: Pathfinders, Stealth Suits, Crisis Suits, Vespids and more. I read somewhere that it's a gyroscope stabilising weapons and armour. The technique is based on Angel Giraldez's Gold NMM recipe. Full credit to him for introducing me to the NMM world with his guides!