This is a rosin spreader from Cecilia, an American rosin manufacturer. In the West, it's becoming a trend to use rosin spreaders not just to apply rosin, but to distribute it evenly on the bow hair. Depending on bow pressure and playing style, the rosin doesn't always contact the strings on the side where it was applied. Using a rosin spreader to spread the rosin evenly to the back of the hair allows for stable contact regardless of playing style. If you're interested, please search for "Rosin spreader". While originally developed for evenly distributing rosin on violin, viola, and cello bows, it's also effective for double bass. When using high-viscosity rosin, such as for pop music, clumps can form on the bow hair. The rosin spreader can eliminate these clumps without damaging the hair, dispersing the rosin effectively.