You do make two extra payments per year, but you only make one extra full payment per year. When you pay bi-weekly, you are actually paying 1/2 of a monthly payment every two weeks. So if your mortgage is $1000 per month, you pay $500 every two weeks. But how the calendar works out, you will have two months out of every year where three payments (of $500) will fall in the same month. So 2 x $500 = $1000 or one full monthly payment.

Regardless, its a good deal to do, especially if you are not disciplined enough to tuck extra money back every month to apply toward the principal. I think the program shaves off about 7 years from a 30-year mortgage.