Nina understood after all her previous experiences that her body was incredibly reactive to anything Kafka said harshly or in a strong manner.
She didn't know if it was because of her blood that she craved a strong man who could control her wild desires with his absolute dominance or if it was because she was a pervert who simply wanted to be crushed by someone she desired with all her heart.
But she couldn't deny that her body was likely to follow Kafka's words more than hers in a crucial situation.
Kafka also knew about this and decided to use this vulnerability of hers to his advantage by directly commanding Nina's little pussy into submission.
He was still a little doubtful if it would work out since he had never seen someone with such a reactive body that seemed to have a mind of its own.