Reverse grinding is when you go behind a couple and gyrate your ass in the same circular motion as the couple or turning your back to the couple and gyrating your ass in the opposite direction.
Reverse grinding happens at every school dance
When a girl is dirty dancing on you ass to ass
Saleem: " Man, that girl was all over me
Wagnus: "But she was facing the opposite way to you , you sure you didn't just bump into her?"
Saleem: nah reverse grind mate