Neon Chaos explains:
To celebrate the Release of Samba De Amigo in Japan, the “Samba Grand Prix” download places a special entrance to Twinkle Circuit in Station Square over behind my the place where you would take the elevator to get in Twinkle Park. When you enter it, you are taken to a special Samba GP Mini game! It’s a whole new twinkle circuit track, with different music from Sonic R, “Everybody’s Super Sonic Racing” instrumental version! Download it now at the SA homepage! Once you beat it, your time will be saved and you can register your time on the Sonic Adventure homepage. This is one of the best downloads yet.