A more secure sheepshank knot...

The sheepshank knot is used to shorten a length of cord that is too long for your needs, where you don't want to cut the cord to size, so you can still retain the longer section for later use.  The basic sheepshank needs to be kept under tension otherwise it comes apart.

This paracord example is a more secure modification that has the working strands tucked through the loop ends of the sheepshank, then the knots are tightened, as you make sure the three cords between the knots are equal in length to share the load you'll be putting on them.

Good diagrams for the sheepshank knot can be found in 'The Directory of Knots' by John Shaw, and ABoK (The Ashley Book of Knots) also has a few variations.

An example use would be to shorten up a tent guyline or tarp tie down, so that any excess cord wouldn't be hanging down on the ground or in the mud, or otherwise in the way.  A fisherman's bend (also called an anchor hitch) is used where the paracord is attached to the tarp grommet.

This sheepshank was a knot I learned in the Boy Scouts (the basic version), but not one often used.  The above demonstration link to the Animated Knots by Grog website mentions alternative possibly more secure knots to use.  I have used the modified sheepshank knot to shorten a long extension headphone cable for my desktop computer, but just coiling it and adding a rubber band would have been easier and faster, lol...