Sometime when rigging an anchor, it’s helpful to shorten a sling by an inch or so to get better sharing of the load.
Example - you have two identical single runners. You planned to clip each one to two different bolts, hopefully resulting in a redundant anchor with the forces equally distributed to each bolt. But, when you get to the anchor, you find that one bolt is a couple of inches higher than the other one, which would mean no load sharing at all if you used two same length runners. With this trick, you can shorten one a couple of inches to make your anchor more equalized.
(You could tie an an overhand knot in the sling, but doing this has a few drawbacks. It weakens the sling a bit, it's tough to untie after it's been loaded, and you can't fine tune the length.)
Instead try one of these two simple tricks. Either one will shorten a sling with each twist or wrap, allowing you to fine tune the runner length by a cm or so per turn.
1 - Add twists to the sling
2 - Wrap one strand a few times around the carabiner
Above we see the "normal" sling with no twists. This stretches to about 21.5 inches.
Twist the sling about 10 times. Now it's shortened to about 19.5 inches, or 2 inches shorter than the "normal" sling.
For this one, wrap one strand of the sling a few times around the carabiner. Four wraps shortened the sling to about 18 inches, or approximately 1 inch shorter per wrap.