Ah I see...
the whole section should read like this:
    if (port == null) {
            String portProperty 
                    = System.getProperty(
                        NinjaJetty.COMMAND_LINE_PARAMETER_NINJA_PORT);
            
            port = portProperty;
        }

That's of course not obvious. But @parameter (in the comments section above) makes it possible to use <port>8081</port> inside the pom.xml configuration of ninja-maven-plugin. And only if it is not already configured there we want to overwrite it with a possible value from the command line.

Just ask if that sounds strange...

Here is a bit more about @parameter: http://maven.apache.org/guides/mini/guide-configuring-plugins.html