It happens when the old man is struggling to reel in the enormous marlin. He's exhausted, and the marlin, still alive, drags him across the ocean for days. He uses his left hand to hold onto the line, his right hand to manipulate the winch.
Here's why it might be seen as a betrayal:
* Physical Strain: The old man's left hand is overworked and cramped, experiencing immense strain. It's a testament to the relentless strength of the marlin, and the way it's pushing him to his absolute limit.
* Loss of Control: Despite his best efforts, the old man loses control over the line. The marlin pulls him further and further, dragging him towards exhaustion and defeat. This loss of control, a result of the immense pressure on his left hand, can be seen as a symbol of his own will being overridden by the fish's strength.
* Symbol of Nature: Throughout the story, the marlin represents nature's power and beauty. The old man's left hand failing him can be interpreted as a metaphor for nature's relentless power and the limitations of human strength against it.
However, it's important to note that this is an interpretation. The text doesn't explicitly state the old man's left hand "betraying" him. It simply highlights the extreme physical challenge he faces and the consequences of pushing himself to the brink.