Let's say you buy 125 points with an April Use Year (UY) and want to book a 200 point stay for November.
11 months before, in December, you log onto dvcmember.com to book your stay. At that moment in time, you have 0 points available. But the system is smart enough to know that you have 125 points coming in April. It automatically assumes you want to use those 125 points and asks you if you want to borrow another 75 from the year after that.
It's important to know that once you borrow points, you cannot "unborrow" them. They have to be used in that year. With an April UY, that means they have to be used by March 31. Otherwise, they are gone even though you still have to pay for them.
The danger in borrowing is that if your vacation plans change, you need to either book another vacation quickly or rent them out to someone on short notice, which usually means at a lower price.
Remember,
DVC is a forced vacation scheme. If you combine banking and borrowing, you can vacation as infrequently as once every three years. But you can't accumulate points indefinitely. You will have to use (or sell) them at least once every 3 years.
It's also possible to transfer points to another DVC member but banked & borrowed points cannot be transferred.
Please let me know if you have other questions.