The default right mouse button option, Select pairs, allows you to select a pair of lines. The Equal Length constraint ensures that the selected lines are the same in length. The right mouse button option, Select multiple, allows you to select a continuing series of lines. In the series, after you have selected two lines, a constraint is created and then the second line you selected is used as the first line for the next pair. This continues until you either use the right mouse button to start a new series or return to standard pairs selection. You can preselect a series of lines before selecting the tool and they are treated like a series selected in 'Select multiple' mode.