With Offline Mode, features that do not require a connection to WordPress.com servers can be activated.
This plugin is using the jetpack_offline_mode filter to enable the Offline Mode. If you are familiar with ftp and file edit, you can also add the define( 'JETPACK_DEV_DEBUG', true); to the wp-config.php file. Make sure you add it before the /* That's all, stop editing! Happy blogging. */ line.
This plugin also checks if the Jetpack plugin is already activated, so it is safe to use this plugin.