A biweekly mortgage pays a 30 year loan off in 22.6 years. It does that by forcing the borrower to make an extra monthly payment annually. Just as when you are paid by your employer biweekly, every few months you are paid three times, instead of twice (because there are more than 28 days in a month), with a biweekly mortgage every few months you are paying it three times instead of twice. In a year you will make a full extra monthly payment.
We find that if you just send in 1/12 of your monthly payment as an extra principal payment monthly, it is a lot less banking to manage then biweekly loans with 2-3 payments a month.