If you pay off principal early, you can aim for more breathing room each month or finish sooner. With the same fixed rate and no fees, keeping the payment usually saves more interest because the balance falls faster.
An example with numbers
Picture a $120,000 mortgage over 20 years at 3%. The starting payment is about $666. If you add $100 a month to shorten the term, that $100 speeds up repayment. If you choose a lower payment instead, the benefit shows up as a smaller monthly obligation.
Try it with your numbers
Enter the same extra amount in both modes and compare the end date, the payment and the interest. Add your contract's prepayment fee if there is one.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.