When you make an up-front cash payment to reduce your monthly payments on a mortgage loan, it's called a buydown.
In a temporary buydown, your payments during the buydown period are calculated at a lower interest rate than the actual rate on your loan, which makes the payments smaller.
For example, if you prepay $6,000, your rate might be reduced by a total of six percentage points, or one percent for each thousand dollars, spread over three years.
Instead of an 8% rate in the first year, it would be 5%. In the second year, it would be 6%, and in the third year 7%. On a $100,000 loan with a 30-year term, a reduction from 8% to 5% would reduce your monthly payments in the first year from about $734 to about $535.
The extra cash you prepaid would be used to make up the difference between the amounts due calculated at the lower rates and the actual cost of borrowing -- in this case about $200 a month in the first year. Then, in the fourth year, you would begin to pay at the actual loan rate and your payments would increase.
In a permanent buydown, which is less common, your rate might be reduced by about 0.25% for each thousand dollars, or point, you prepaid, but the reduction would last for the life of the loan.
You might choose to do a buydown if you had extra cash at the time you were ready to buy, but a smaller income than would normally allow you to qualify to buy the home you want.
In most cases, lenders require that your housing costs be no more than 28% of your income. You might be able to reach that level if your initial payments were less at the time of purchase. In other cases, a home builder who is having trouble selling new properties might offer buydowns through a local lender to encourage reluctant buyers to take advantage of lower payments in the first years they own their homes.
For helpful information on 2/1 Buydowns, check out Justin McHood's article on the subject on Porchlight, Zillow's blog.