To the layman's eye, mortgage rates seem to move up and down without explanation. But just like the ocean tides that wash up and back by the pull of the moon's gravity, mortgage rates have their own driving force, even if they have a less cosmic source.

The mortgage rate basics

The mortgage lender that funds your loan is called the originator. A loan originator may be a bank, credit union, or other type of financial institution. On the date of funding, the money flows out of the originator's hands and into yours. You then turn that money over to the seller of the home.

Once the loan is funded, the originator has the option of keeping that loan in its portfolio or selling it on the secondary market. If the originator keeps the loan, it makes money by way of the interest you pay each month. If the loan is sold, the originator replenishes its funds and can make more loans to other homebuyers. Basically, the secondary market investors keep funds circulating so that loan originators don't run out of money for new mortgages.

Who are these mortgage interest rate folk?

Today's secondary market investors include government-chartered companies like Fannie Mae and Freddie Mac, plus insurance companies, pension funds, and securities dealers. Although Fannie Mae and Freddie Mac are different organizations, they participate in similar activities. Both can buy mortgages, and both can group mortgages together for resale in what's called mortgage-backed securities. These are highly liquid investments, meaning that they can be readily bought and sold.

Investor demand

Here's how the secondary market affects you as a would-be homebuyer. Investors want to earn the best return possible. That level of return is determined by the current and anticipated condition of the economy. When the economy is on an upswing, future yields are expected to be better than current yields. Investors, therefore, will hold off buying until higher yields materialize. This drives mortgage interest rates up, because lenders cannot sell their loans at lower yields.

Conversely, when the economy is in a downturn, investors buy up what's available to avoid being stuck with lower yields later. This drives mortgage rates down, as investors are clamoring to buy before yields get too low.

What it means to you

By staying on top of financial trends and planning accordingly, you can time your rate lock to compare and get the best mortgage rate possible. In other words, when the tide is low, put a call into your lender and lock in that rate. You'll enjoy waves of prosperity if you do.