How does Rainsford manage to wound Zaroff?My question is about this passage: "I will not lose my nerve. I will not." He slid down from the tree...

Rainsford manages to hurt Zaroff just a little bit by setting a trap for him.

Specifically, Rainsford uses a big tree that is leaning on a little one.  We are not told exactly how he manages it with only a knife as a tool, but he is able to make it so that the big tree will fall when Zaroff touches a trigger (a protruding branch).  Zaroff touches the trigger and the tree falls.  But Zaroff is quick enough and is paying good attention so the tree only hits his shoulder a little.