Someone on the boards presented it like this for me a long time ago, and it really helped it click. You can lay it out like this with your points, regardless what UY. I'll use Aug, since that is our UY.
How points work (unless banked):
* 2007 points were deposited on Aug 1, 2007, and were good for vacations from Aug 1, 2007 - July 31, 2008.
* 2008 points were deposited on Aug 1, 2008, and are good for vacations from Aug 1, 2008 - July 31, 2009.
* 2009 points will be deposited on Aug 1, 2009, and will be good for vacations from Aug 1, 2009 - July 31, 2010.
Banking and some things to remember:
* For any UY, 100% of points can be banked during the first 8 months of your UY. Since we have an Aug UY, we can bank points starting Aug 1 and bank through March 31 of any given year.
* Points can be banked into the next UY. When you bank, you may use those points for vacations scheduled within the next UY. For example, if I bank points from 2008 into the 2009 UY, I have until March 31, 2009 to do so (first 8 months of my UY). If I bank 160 points by this coming March, I can use those points to book vacations from Aug 1, 2009 - July 31, 2010.
* Once banked, points cannot be banked again. Those 160 points must be used by July 31, 2010, or they will expire.
* Once banked, points cannot be returned to their original UY.
* It's common practice that MS uses banked points first when making reservations, but I always ask. Just makes me feel better!

Also, if you happen to cancel part of a reservation, make sure what is credited back to your UY is your current UY points, not your banked points. Again, should be standard practice, but sure doesn't hurt to ask.
I hope this helps. It really helped me when I was trying to understand the whole banking/borrowing thing!
