As with all Maven plugins, log messages produced by the plugin itself will be sent to stderr and only at the INFO level and above. To see DEBUG level messages from the plugin, invoke the plugin with the -X flag:
mvn -X jetty6jetty:run
Log messages from the Jetty instance embedded in the plugin are controlled in much the same way as Jetty standalone. The plugin picks an appropriate JSP version based on the version of the jvm executing the plugin. Please see the Jetty Maven2 Plugin pages for an explanation of how to configure logging.