When thinking of banking and borrowing, you have to think of it as a yearly alotment and a way to extend that allotment. One year's points not used in that year can be banked into the next use year for extednded use. In the same way, you can borrow from a next use year and use in this year. After banking one year into the next, the points will expire if they are unused.
Example:
Say (for ease in numbers) you have 100 points with a March use year. That means they are available for vacations that occur between March 1, 2007 and February 29, 2008. You decide you do not want to vacation this year, so you "bank" those points into your next use year. That means that now instead of 100 points to use between March 1, 2008 and February 28, 2009, you will have 200 points at your disposal. If you can use at least 100 of those points in that use year, you will be fine, but any less than that, and you will lose points. I always think the easy way to think of it is to make sure you use at least as many points as you bank in the year you have banked into. Any others from that year can then be banked into the next. We are usually in a constant state of banking, although we did borrow from our AKV use year this time.