At any time during your use year, you can borrow up to 100% of the following use year's points. Borrowing is done when you make a reservation that you do not have current use year points to cover, so when you borrow, you only borrow the exact number you need for that reservation.
At any time during the first six months of a use year, you may bank up to 100% of that year's points to the following use year. The points then become available at the start of the next use year and are available for the entire use year.
By the end of the ninth month, you can bank a cumulative total of up to 50% of your points .
By the end of the tenth month, you can bank a cumulative total of up to 25% of your points.
In the final two months of your use year, you may not bank.
Once you move points, you can't move them again. So if you borrow points, you can't unborrow them, nor can you bank them. If you bank points, they are gone for the current use year, cannot be moved back or borrowed back, and they don't become available until the begiinning of the following use year. Banked points also cannot be banked forward another year; however, I believe they can be banked into II (not sure about that, or any limitations/restrictions which may attend to that).
If I've failed to confuse you, please let me know.