Apparently, the answer is . . . . NOTHING!
This rider was at a show jumping competition in Bourg-en-Bresse, France. The class was supposed to a costume class, but the rider D. Ribot did not have a costume ready. The judge told him to compete naked (we can assume it was a joke). He did.
Although I cringed for him over some of the landings, it’s an impressive performance. Now, can he still father children?