The Old Man and the Sea: How did Santiago finally kill the marlin?

Santiago harpoons the marlin through its heart.

The old man dropped the line and put his foot on it and lifted the harpoon as high as he could and drove it down with all his strength, and more strength he had [93] just summoned, into the fish’s side just behind the great chest fin that rose high in the air to the altitude of the man’s chest. He felt the iron go in and he leaned on it and drove it further and then pushed all his weight after it.