They went underground during the housing bust, but home flippers are now making a comeback. And this time, they may be helping the housing market recover instead of destabilizing it.

The research firm RealtyTrac recently issued its mid-year flipping report, which shows that investors flipped more than 136,000 single-family homes during the first half of 2013. That’s 19% higher than the pace last year and 74% percent higher than in 2011.

Related: This is Housing Bubble 2.0: David Stockman

Flippers—defined as people who buy a home and then sell it within six months—have made an average gross profit of $18,391 on their investments so far this year, compared with just $5,321 during the first half of 2012. During the same period in 2011, flippers lost an average of $13,206.

Flippers make housing experts uneasy because they’re speculators who can distort normal supply and demand dynamics. They contributed to the housing bubble from 2004 to 2006 by helping generate a frenzy of demand in some areas, one reason prices rose well above fundamental levels. Many flippers lost a bundle when the housing market crashed.

This time, however, flippers may be aiding the housing recovery, as Aaron Task and I discuss in the above video. Many flippers these days are professional investors who have pooled funds of money so they can buy distressed properties, quickly fix them up, and sell them for a tidy profit just as buyers are wading back into the market. Many individuals who may have been able to get financing for flips during the bubble can’t get a loan these days, since banks are still very reluctant to finance speculation.

Related: This Housing Recovery Has Legs: Michael Santoli

Here’s one sign that flippers may be helping the housing market heal: Flipping activity is actually declining in several markets hit hardest by the housing bust, such as southern California, Phoenix and Las Vegas. That’s probably because buyers have already sopped up many foreclosed homes and other distressed properties in those markets. Getting such homes off the market puts a floor under prices, which ought to help lure more sellers to list their homes, since they won’t be competing with distressed merchandise offered at bargain basement prices. Real-estate agents in many areas say the biggest problem these days isn’t a lack of buyers but a lack of inventory, since many owners are still worried about taking a loss if they try to sell.

There are a few surprises, meanwhile, among the top 15 markets for flippers so far this year. Florida remains a flipper’s paradise, with 7 cities among the top 15, but Pittsburgh, Cincinnati, Omaha and Charleston, S.C. also make the list. That’s a sign of robust activity in normally staid markets. Chicago, Washington, D.C. and metropolitan New York City also made the list, a sign there’s money to be made in reasonably strong markets that didn’t suffer terribly during the bust.