The way it works is once you sign up ( there is usually a fee involved around $300 or so ), they begin pulling your mortgage payments from your checking account on a biweekly basis. It does save money, because essentially you are making half of your payment 2 weeks early and not having to pay that extra interest.
It does not free up any money for you right now though. You still make the same payment, except its broken into 2 smaller ones instead of one large one.
Depending on your mortgage, you could save anywhere from $10,000 or much more. You could also set this up on your own though ( as long as you are disciplined enough to actually make the payment on your own ) and then you wouldn't have to pay that initial fee.
We don't do it on our mortgage, but I am very familiar w/ the procedure from when I worked at the bank.