What is the plot twist in the resolution of "The Most Dangerous Game"?

This question could be referring to two points in the story.  It depends on exactly which part of the story a reader considers the resolution.  

The biggest plot twist occurs after Rainsford hurls himself off the cliff in order to escape from Zaroff.  

Twenty feet below him the sea rumbled and hissed. Rainsford hesitated. He heard the hounds. Then he leaped far out into the sea. . . .

Thinking that he no longer has...

This question could be referring to two points in the story.  It depends on exactly which part of the story a reader considers the resolution.  

The biggest plot twist occurs after Rainsford hurls himself off the cliff in order to escape from Zaroff.  

Twenty feet below him the sea rumbled and hissed. Rainsford hesitated. He heard the hounds. Then he leaped far out into the sea. . . .

Thinking that he no longer has to worry about or hunt Rainsford, Zaroff returns to his house for dinner.  He's a little bothered by the fact that Rainsford escaped and Ivan is dead, but Zaroff still takes comfort in his food and wine.  At ten that night, Zaroff heads to bed.  This is when the first plot twist occurs.  Rainsford is hiding in the bedroom.  The plot twist is that Rainsford has now become the hunter and Zaroff is the prey.  

A man, who had been hiding in the curtains of the bed, was standing there.

"Rainsford!" screamed the general. "How in God's name did you get here?"

"Swam," said Rainsford. "I found it quicker than walking through the jungle."

The general sucked in his breath and smiled. "I congratulate you," he said. "You have won the game."

Rainsford did not smile. "I am still a beast at bay," he said, in a low, hoarse voice. "Get ready, General Zaroff."

Readers are not told exactly what happens, but we presume that Rainsford kills Zaroff because we are told that Rainsford sleeps very soundly in Zaroff's bed.  

He had never slept in a better bed, Rainsford decided.

This isn't as definite of a plot twist as the previous twist, but the fact that Rainsford sleeps in that bed and sleeps well might indicate that he is (will become) the next "caretaker" of the island.  Some readers interpret this conclusion to suggest that Rainsford, having experienced the thrill and excitement of hunting humans, can no longer be happy hunting anything else.