This is a set of optional parts for Mini 4WD car models.
From the manufacturer:
Your machine scaling the circuit fence after landing can be a frustrating way to lose a race, and the under guard reduces the chances of this happening. Made from a low-friction resin material, it also helps your Mini 4WD racer keep its momentum through tricky grass sections. This cool black 3-piece part is compatible with both normal and wide roller set-ups. Small parts for attachment are also included.
Compatible Cars: AR, FM-A, MA, MS, VS, Super XX and Super-II chassis cars