This plugin listens for comments on pull requests and will trigger a GitHub multibranch job if a comment body matches the configured value, such as "REBUILD". This is implemented as a branch property on multibranch jobs.
To enable this behavior, simply add the branch property to the multibranch job and configure the string to use. Make sure that you have a GitHub server properly connected via the Jenkins configuration, or else this will not work.
Use the instructions found in this wiki page.