Gradebook

I'm using Moodle 1.9. In gradelib.php I have noticed a method called "grade_update_mod_grades". In the comment it says that the method forces a full update of module grades in central gradebook - works for both legacy and converted activities.

I don't understand what the method is doing. When is this method used? Is it called automatically out of Moodle or manually?

Thanks for a response