Rainsford is being hunted by Zaroff and to escape the hunter he climbs a tree. Zaroff stops and his eyes climb up the trunk of the tree. He smiles and blows a smoke ring toward the tree. Then Zaroff goes off in the opposite direction.
"Rainsford held his breath. The general's eyes had left the ground and were traveling inch by inch up the tree. Rainsford froze there, every muscle tensed for a spring. But the sharp eyes of the hunter stopped before they reached the limb where Rainsford lay; a smile spread over his brown face. Very deliberately he blew a smoke ring into the air; then he turned his back on the tree and walked carelessly away, back along the trail he had come."
“Rainsford's second thought was even more terrible. It sent a shudder of cold horror through his whole being. Why had the general smiled? Why had he turned back?”
“Rainsford did not want to believe what his reason told him was true, but the truth was as evident as the sun that had by now pushed through the morning mists. The general was playing with him! The general was saving him for another day's sport! The Cossack was the cat; he was the mouse. Then it was that Rainsford knew the full meaning of terror.”
The smile is what makes Rainsford believe that he is being played with.
Because general Zaroff blew smoke right at him up in the tree where rainsford was hiding.