A type of prepayment penalty: the borrower is required to pay a prepayment fee equal to the difference between the original interest rate and the current market rate multiplied by a set penalty amount. This calculated amount is meant to equal the total return that a lender would receive on the loan if it were carried for the entire term.

Bankapedia's Take

You won't see yield maintenance on residential loans; at least it won't be referred to as such. Yield Maintenance is common in commercial mortgages and is one of the reasons that commercial mortgage refinancing is not nearly as prevalent as residential. Imagine wanting out of your existing mortgage after 3 years, but your yield maintenance carries a penalty through the first 10. Meaning the bank can charge you the 7 years of unpaid interest. Ouch