What trap does Rainsford put together next using his knife? what is the result?

"He thought of a native trick he had learned in Uganda. He slid down the tree. He caught hold of a springy young sapling and to it he fastened his hunting knife, with the blade pointing down the trail; with a bit of wild grapevine he tied back the sapling. Then he ran for his life." The trap kills Ivan and not Zaroff.