if in the example of 100,000,000 the 5% is due between July 1 and the next June 30 and any of it is paid before July 1 - or before the amount due by dividing the year by 360 or 365 days - then such payment exceeds interest due at that point in time and the principal is reduced by that excess amount
there would need to be a lot of pre payment to be meaningful I think