Rates for 30-year fixed mortgages jumped half a percentage point this week thanks to the ongoing upheaval on Wall Street. For anyone trying to time a refinancing or new mortgage, it's been a wild ride.

The average interest rate is 6.46 percent, according to the latest survey by Freddie Mac, up from 5.94 percent last week. So far this year, rates have swung between a low of about 5.5 percent and a high of more than 6.6 percent, its survey shows.

Bankrate.com, which does its own survey, saw a similar increase this week -- the largest in more than 21 years, according to its records. (The last time rates rose by more than half a percentage point, they were hovering around 10 percent.) Bankrate.com offers several explanations for the volatility, including this one:

Cameron Findlay, chief economist for LendingTree.com, says the roots (sorry) of the increase in mortgage rates lie in technical matters. The money in a mortgage-backed security goes in three directions: the investor, Fannie Mae or Freddie Mac, and the servicer who handles billing and collections. Lately, as prices for mortgage-backed securities have plummeted, investors and servicers have been squeezed -- so they demand higher coupon yields and therefore higher mortgage rates.