What happened when the fish lurched?

From the text:

Just then the fish gave a sudden lurch that pulled the old man down onto the bow and would have pulled him overboard if he had not braced himself and given some line.
The bird had flown up when the line jerked and the old man had not even seen him go. He felt the line carefully with his right hand and noticed his hand was bleeding.
“Something hurt him then,” he said aloud and pulled back on the line to see if he could turn the fish. But when he was touching the breaking point he held steady and settled back against the strain of the line.