Why does Jim grab “the light cord” at the last minute?

Instinct and curiosity.....

At length I was clear of my dangerous neighbour, and just as I gave the last impulsion, my hands came across a light cord that was trailing overboard across the stern bulwarks. Instantly I grasped it.

Why I should have done so I can hardly say. It was at first mere instinct, but once I had it in my hands and found it fast, curiosity began to get the upper hand, and I determined I should have one look through the cabin window.