A mortgage rate lock is simply MSUFCU promise to hold a certain interest rate for you for a specified period of time before you close on a loan. Because interest rates have the capability of fluctuating on a daily basis, you will need to consider the costs you may pay if a rate increases or the expense of paying to lock in a lower rate. Once you close on your mortgage, your rate stays the same, unless you have a variable rate loan, in which your rate adjustments would be specified. New home purchase: The interest rate for a purchase is locked in once we receive an accepted offer to purchase a home. Refinance: Your rate is locked from when you apply when refinancing your home.