How does making biweekly or extra principal payments actually save mortgage interest?

Paying biweekly means you make half your monthly payment every two weeks, which adds up to 26 half-payments, or 13 full payments, per year instead of 12. That one extra payment goes straight to principal, shrinking your balance faster and reducing the interest charged on it over time. On a typical 30-year loan, this can shave several years off the term and save tens of thousands in interest. You can achieve the same effect by simply adding a bit extra to your regular monthly principal. Confirm your servicer applies extra funds to principal, not next month's payment, and check for any prepayment quirks, though federal law bans prepayment penalties on most owner-occupied loans. No special program needed; just pay a little more.