You can only borrow points from a subsequent use year. For example, let's say your use year is May.
Your 2001 points can be used for a reservation between May 1, 2001 and April 30, 2002.
If you borrow your 2002 points, they must also then be used for a reservation bewteen May 1, 2001 and April 30, 2002.
If you want to make a reservation between May 1, 2002 and April 30, 2003 (this is your 2002 use year and you have no points left from 2002 because you used them in 2001 use year), you can borrow points from your 2003 use year.
If you keep this up, sooner or later, you're going to have to sit a year out or buy more points!
You don't have to wait until your use year begins to make a reservation. Usually, points aren't borrowed until you make a reservation. Use year is ONLY the month of the year that your points renew. It has nothing to do with when you can make a reservation. As long as points will be available at the time of your stay, you can make a reservation.
Hope I haven't confused you further.