You may borrow only from the Use Year following the date of the reservation you are making. Borrowing can only be done when actually making a reservation.
If you have a June Use Year and want to make a reservation for August, 2007 you could borrow from your June, 2008 Use Year if necessary for that stay (at your home resort) and you could call today to make the reservation. For any date in August, 2007 you would be able to use points banked from your June, 2006 Use Year, points from your June,07 UY and points borrowed from your June,08 UY.
You could NOT borrow from your 2008 UY for a reservation prior to June 1, 2007. You COULD borrow from your 2007 UY for a trip prior to June 1, 2007. You cannot borrow from you 2009 UY until making a reservation for a stay after June 1, 2008 ... and that reservation cannot be made until after July 1, 2007 and only for your home resort.