MIDA-609 I was pierced to pieces by the regular, rhythmic pistons of a machine... Nao Jinguji

It's a piston! Not fast...but certainly not slow...getting closer to the climax with every second! Nao Jinguji, who boasts that "sex without variation in pace sounds boring...", is gradually broken down by the steady piston...! She's almost there and wants him to go faster, but it's constant.