when you don't have enough points to made a reservation in this current year - then you will be asked if you want to borrow.
You can't borrow unless and until you need the points to make a reservation.
use year is generally not an issue - however if you are borrowing at the end of your use year - MS will make sure you understand what cancelling means to borrowed points - they can't go back to the year that you borrowed from - they must be used in your current year.