It usually works with payment option adjustable-rate mortgages where the borrower has a cap on the interest amount paid in case the interest rate increases.
If the interest rate increases, the borrower can make a minimum interest amount payment according to the cap.
The rest of the interest amount gets added back to the principal balance increasing the original amount that you borrowed.
This will increase the monthly mortgage payment for the borrower and in some cases could owe more than what the property is worth.
In another case, if the borrower is unable to make a payment and the lender is due to any financial hardship, the lender can offer a deferment option where the borrower can stop making the payment of the loan for some sometime.
By doing that the lender will accumulate the interest rate and add it to the outstanding mortgage balance.
This will increase the principal amount and the borrower will be making higher monthly payments compared to standard payments.
An example of Negative Amortization is Jane taking a mortgage of $200,000 for 30 years to buy a new property at an interest rate of 5.5%.
According to the mortgage agreement she is allowed to make a minimum payment of $800 for a certain period of 30 years.
After that period is over, she needs to make regular payments of $916.67.
Due to some financial hardship, she only opts to pay the $800 instead of $916.67 as per the mortgage agreement.
This means that she is making $116.67 less towards the interest every month. This deficit interest amount is added to the principal balance of $200,000 by the lender and for the next repayment cycle, the new principal balance would be $200,116.