Borrowing of points happens when you go to make your reservation.
Once you know what UY your trip will happen, you can easily figure out what points can pay for them.
For example, 2011 UY points can be borrowed to "pay" for any trip that you take during your 2010 UY. With October, trips from October 1st, 2010 through September 30th, 2011 would be eligible.
Any trips during this time can use 2010 UY points, banked 2009 UY points, and borrowed 2011 points.
You can still make your reservation 11 months in advance of travel, regardless as MS looks at what will be available at the time of the trip, not what is available when you call.
Good luck!