WASHINGTON — Though there are still some snares and drawbacks for participants, one of the federal government’s most important financial relief efforts for underwater homeowners started operating Nov. 1.

It’s a new short-sale program that targets the walking wounded among borrowers emerging from the housing downturn — owners who owe far more on their mortgages than their current home value but have stuck it out for years, resisted the temptation to strategically default and never fell seriously behind on their monthly payments.

Industry estimates put the number of underwater owners across the country at just under 11 million, or 22% of all homes with a mortgage. Of these, about 4.6 million have loans that are owned or securitized by Fannie Mae or Freddie Mac. Eighty percent of these Fannie-Freddie borrowers, in turn, are current on their mortgage payments and meet the baseline eligibility test for the new short-sale effort.