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 only on the surface where it's 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." Also, although originally developed to evenly distribute rosin for violins, violas, and cellos, it's also effective for double basses. When using high-viscosity rosin, such as for pop music, clumps can form on the bow hair, but the rosin spreader can eliminate these clumps without damaging the hair, dispersing the rosin effectively.