As this knot is tightened the red loop grips the green loop, and also traps the short end of the red loop. The green loop (or bight) must be squeezed together very tightly to avoid slippage ~ the red loop tightens on the green loop to enable this. For most cordage the friction is sufficient to guarantee a good hold. The knot cannot be trusted with smooth materials, such as fishing line.
To maintain the holdfast of the knot it does have to remain under tension. The loosely tied knot in the picture above would unravel easily with a few shakes. This would also be the case if the tension varied. For example a small boat ~ secured with two ropes joined in this way ~ will strain and relax against the moorings as waves cause it to surge to and fro ~ the knot will loosen ~ the boat will come adrift.
We find the sheet bend to be an excellent knot for joining short lengths of string when tying a parcel ~ a good economy measure. It only takes a few moments to add to the security by making a hitch with the two loose short ends around the straining lengths ~ causing a horrible parcel knot, but one that has withstood all the journeys in which it has been used. It is not the most useful knot when straining the string to make it very tight ~ but I find there are not many knots that are.
Our main use for this knot is when tying the hanging string on the back of picture frames. Tension in the string remains steady, and we usually secure the ends and tidy things up by applying some tape. I find it to be neater than the usual knots tied by other framers whose work I have witnessed ~ but it is out of sight so that is not an important comment.
The sheet bend is fairly streamlined, which might be useful when cordage has to be threaded through holes, especially if the loose ends are taped to the standing parts. This is only a surmise ~ I have no experience of it.
