Not P but Moby-Dick (98)

[Mike Jing]

>From Chapter 126:

The life-buoy—a long slender cask—was dropped from the stern, where it
always hung obedient to a cunning spring; but no hand rose to seize it, and
the sun having long beat upon this cask it had shrunken, so that it slowly
filled, and the parched wood also filled at its every pore; and the studded
iron-bound cask followed the sailor to the bottom, as if to yield him his
pillow, though in sooth but a hard one.

What is this "cunning spring" exactly? Is the "spring" a metal coil or a
nautical "spring line"?