This is a very handy process to be acquainted with if you’re making user exemptions/overrides for a quiz to accommodate students or just to differentiate assignment rules for students who might need more time, an extension or retake. User overrides are handled at the quiz level and can be a valuable course administration tool. The video was created by BBCS eLearning and they have a great collection of Moodle videos and tutorials on Youtube: http://www.youtube.com/user/BBCSeLearning
Learn more about override permissions in Moodle here: http://docs.moodle.org/25/en/Quiz_settings#Group_and_User_overrides
Direct video link: http://www.youtube.com/watch?v=UXk2010JuzU