What does Zaroff mean when he nods to Ivan?

Zaroff is inferring that Rainsford can play his game..... or be dealt with by Ivan.

But may I not venture to suggest that you will find my idea of sport more diverting than Ivan's?"

He nodded toward the corner to where the giant stood, scowling, his thick arms crossed on his hogshead of chest.

He is giving Rainsford the choice to join his hunt or be hurt by Ivan, which is the choice he gives his other victims.