Right, it is a rolling period so the 5 year window moves from one day to another all the time. Once you have been a PR for more than 5 years, you must always have 730 days in Canada looking back at the past 5 years from any date.
If someone landed in June 2007 and they met the residency requirement for their first 5 years, they must now be aware that the days they stayed just after their landing will be moving outside that past 5 year window. Say if they count towards their 730 days some days they stayed when they landed, if they leave Canada at the 5 year mark, these days they stayed after landing move outside the 5 year window and if they were at exactly 730 days, they will move under the 730 day mark. If they have a valid PR card, they will probably get into Canada just fine and meet the requirements during their next PR card period. However, if something happens to the PR card, maybe it is lost, stolen or destroyed and the person needs to apply for a PR travel document to return, if they do not have 730 days in Canada in the past 5 years at that point, they may not be given a travel document and could lose their PR.