Normally, the mortgage payment you make to a lender has two parts: interest due the lender for the month, and amortization of principal.
The real purpose of negative amortization has been to reduce the mortgage payment at the beginning of the loan contract. It has been used for this purpose on both fixed-rate mortgages (FRMs) and adjustable rate mortgages (ARMs). A second purpose, applicable only to ARMs, has been to reduce the potential for payment shock, a very large increase in the mortgage payment associated with an increase in the Adjustable Rate Mortgage (ARM) interest rate.
Negative Amortization - Downside
The downside of negative amortization is that the payment must be increased later in the life of the mortgage. The larger the amount of negative amortization and the longer the period over which it occurs, the larger the increase in the payment that will be needed later on to fully amortize the loan.
On fixed-rate loans, negative amortization is a tool for reducing the mortgage payment in the early years of a loan, at the cost of raising the payment later on. Instruments that incorporate this feature are called graduated payment mortgages or GPMs.