The mechanism, stripped of marketing
A biweekly plan splits your monthly payment in half and collects it every two weeks. Because a year contains 26 fortnights, you make 26 half-payments — the equivalent of 13 monthly payments rather than 12. That thirteenth payment is applied entirely to principal, and it is the entire source of the saving.
What it is worth
On a $340,000 loan at 6.5% over 30 years, biweekly payments typically retire the loan around five years early and save somewhere near $85,000 in interest. That is a real result — but it comes from paying roughly 8.3% more per year, not from any magic in the payment frequency.
The free version
Divide your monthly payment by twelve and add that amount to each monthly payment, marked "apply to principal". The outcome is nearly identical, you keep control, and you pay no enrolment fee. Third-party biweekly programs commonly charge $300–$400 to set up plus a few dollars per transfer — money that would otherwise go to your balance.
Cash-flow fit
Biweekly works especially well if you are paid fortnightly, because the payment lands with the paycheck and two months a year contain three paydays. If you are paid monthly, the extra-payment approach is simpler and produces the same result.
Check the servicer's handling
Some servicers apply each half payment on receipt, which gains a small amount of extra interest saving. Others hold the first half in suspense until the second arrives. Neither is a problem, but the first is slightly better, and it is worth a single phone call to find out which you have.
Where this sits in your priorities
Prepaying a mortgage is a guaranteed return equal to your rate. It should come after high-interest debt, after an emergency fund, and after capturing any employer retirement match — but for many households with a rate above 6%, it is a perfectly good place for surplus cash.