DT Swiss Pro Head Reinforcement Nipple Washer

From DT Swiss:

Essential for rebuilding DT's premium PHR system rims
• Only to be used with DT's dedicated Squorx Pro-Head nipples
• These specially shaped washers spread the load across the rim and allow the convex Pro-Head nipples to pivot freely in order to align perfectly with the spoke, creating a stiffer, stronger wheel with no weight penalty