The sheet rope can then only fly away so far, with the stopper knot preventing it passing through the lead block. It is quite easy to make, and looks very symetrical when tightened properly. It is reasonably easy to undo too.
1. A loop is made over the standing part
2 Loop the bitter end under the standing part
3. From above pass the bitter end through the first loop you formed.
4 Tighten it right up. This is the whole sequence- Over, Under, Over, Through