video How to Tie a Taut Line Knot
This knot is very useful for adjusting tie downs quickly and easily. For example, a tarp could be held down by a series of these knots and be made very tight so the wind cannot make it rise, and easily be removed simply by sliding the knots later. A taut line knot is also used to keep large tents from sagging by keeping tension on the lines leading to the stakes. A slight variation of this knot is used to climb straight up a rope when doing tree trimming. I currently use two of these knots to hold down a blanket as a seat cover on my truck.
As with any good knot, a taut line can easily be untied when desired. It is certainly one of the more useful knots to have in your skill set.