Knowing Yamaha it was for the extra shove from the tuned exhaust!
"An act done upon some poor unsuspecting male. A person approaches from behind, and uppercuts between their legs with their thumb, pointer, and middle fingers all straight up in a triangle pattern, making sure the thumb contacts the butthole, and the pointer and middle finger pokes both testicles. All while shouting "THUNDERCAAAAAAAAAAT!" as loudly as possible."
Or, maybe not!
Good health, Bill