Guaranteed Return

Good morning! I’m sure many others in the group subscribe to Barry Habib’s MBS Highway. Anyway, on his morning update today, Barry mentioned that making additional payments toward principal is a guaranteed return at whatever the interest rate of the mortgage is. In his example, he said if the rate was 6.5%, then by paying extra, you would have a guaranteed return of 6.5%. I know there was a time when I believed the same thing, but after one of the lessons I learned here, I now believe that paying additional toward principal provides 0% return. Can anyone point to where I’d find that lesson? Also, I’d love to hear what you think as well! Have a great day!