The COC has a 7 day tablet free interval allowing a period to occur.
Taking the COC back to back (starting a new pack of tablets after 21 days, without the 7 day interval) will maintain levels of Oestrogen and Progesterone thus preventing the uterus lining from being shed and postponing menstruation.
This is not an option for a female taking the Progesterone only pill (POP), as they do not have a table-free interval and the customer may want to be referred to their GP for further advice.