You bank any points you don't want to use in your current use year prior to the last four months of that use year. You borrow when you make a reservation and it's based on the reservation date.
Assume your use year is June 1st, you got in the system as of June 1st 2008 and you have 100 points. It is now July something or other 2008 and you want to make a reservation for March 2009 using 75 points.
If your reservation is for your home resort, you are within your 11 month advantage period and you can make it now. If the reservation is for a different
DVC resort, you'll have to wait until the check-in day (March whatever) of August 2008 to call. (You can call later than that; up to a day or two ahead I believe though availability is not guaranteed which is why you'll see most discussions centering around the 11 or 7 month timeframe.) Ok, now you have a March 2009 reservation and 25 points left in your 2008 use year. You can wait until January 31 2009 to bank the points into 2009 (giving you 125 points for a reservation during the June 1 2009 to May 31 2010 timeframe) or you can make another reservation using those 25 points up until May 31 2009. If you don't bank them by January 31 2009 or use them yourself or rent/transfer them to someone else by May 31 2009, they are gone.
If you decide that you would like to use those 25 points during the June 1 2008 through May 31 2009 timeframe, but the only reservation you are interested would be 30 points, you would call (either 11 months or 7 months or less, based on which DVC resort you want) and Member Services will "borrow" the extra 5 points from your 2009 Use Year. So, say you made both reservations tomorrow -- one for 75 points in March 2009 and one for 30 points in, I don't know, September 2008. You've now used up all your 2008 points and have 95 points left for 2009.
You may find it helpful to get a piece of lined paper and put the next 24 months or so on it, then try and note what would happen based on different scenarios. We're used to thinking of a year as starting on January 1st, so the Use Year starting at a different time gets confusing. Also, trying to think about the 11 or 7 months lead time for making a reservation vs. when you have to bank your points can get mixed up in your head. Don't worry too much about the borrowing, except if you start planning a big get-together that might require you banking some points into next year *and* borrowing points from two years ahead. That's when I really reach for the paper so I make sure I'm able to do what I want to do!