The U.S. Rules method (aka Simple Interest) gives the same results as the Normal / Actuarial method except when negative amortization occurs (when payment start date is two or more periods away from the loan start date).

The U.S. Rules method does not charge interest on the interest from negative amortization. The payments of U.S. Rules are therefore lower than the Normal / Actuarial method when negative amortization occurs otherwise they are the same.

In the U.S. Rules Method, periodic payments are applied to interest first. When the interest due is paid off, payments are then applied to the principal.

 

Below is a screen shot of U.S. Rule calculation where there is negative amortization.

Notice the difference in Interest Due and Principal Paid when compared to the normal / actuarial method.

(Schedule columns have been moved for readability.)