The Sheepshank is a shortening knot, which enables a rope to be shortened non-destructively.
The knot is only really secure under tension, it will fall apart when slack. (See tip below.)

Tip. Use up to five half hitches each end of the Sheepshank to make the knot more secure, and for fine tuning the shortening.
Tip. Never cut ropes to shorten them! Always use a shortening knot such as the Sheepshank, or coil the excess.