This plug-in writes a message to the Mantra logfile. The message may consist of multiple lines and may contain variables. For example, to log a start_trial message (assuming that the trial sequence is called 'trial_sequence'), followed by a number of independent variables, you can write the following message:
start_trial [count_trial_sequence] object = [object] orientation = [orientation]
You can find more information about Mantra on http://www.cogsci.nl/mantra.