If you need to undo a quick link at the roadside without a tool, position it on the chainring so it sticks outwards (ie the QL and a normal link form a triangle sticking out over one chainring tooth) then bash the appropriate edge of the QL with a blunt object like the side of a multitool.
With the emphasis on "the appropriate edge". Whether its the nearer or farther edge depends on which side of the triangle is formed by the quicklink.
