The Sew UV command allows you to quickly connect matching pairs of discontinuous UVs. To use the command, select an edge or vertex that is currently discontinuous and press the Sew button. The command will move both matching UVs to an averaged UV coordinate and convert them in to a single UV. This is very useful for mending UV maps that have been broken apart either manually, or via mapping tools such as an Atlas Map projection.
When working with Edge selections you can simply select one of the two corresponding UV pairs to use the command. However, to use the sew command on vertex selections you must have both vertices selected before firing the command.
It is very helpful to massage the UVs somewhat prior to using the command so that the UVs are not too far apart before they are "sewn" back together.
