Something odd happened during U.S. stock trading on Sept. 3: Shares of medium-size companies sank while the rest of the market rose, a finish that occurs about once every 100 sessions.

But to some traders, the unusual move wasn’t a mystery. They had been watching for Good Harbor Financial LLC, a fast-growing money manager that deals in nearly $11 billion of exchange-traded funds, to reshuffle its main portfolio, as it does regularly on the first trading day of every month. That day, Good Harbor sold a big stake in the iShares Core S&P Mid-Cap ETF.

One month later, Good Harbor was buying midcap stocks. That day, midcaps rose faster than other parts of the market in a broad rally. Then on Nov. 1, the Russell 2000 index of small-company stocks fell as other corners of the market rose. This time around, Good Harbor was unloading small caps and buying large-cap stock ETFs.

Across Wall Street, traders take note when big ETF managers like Good Harbor step into the market, moves that can affect prices. At times, they may try to profit by jumping ahead of Good Harbor, potentially chipping away at returns for its investors, people familiar with the market said.

To continue reading the Wall Street Journal article about Gemini and NLD client, Good Harbor Financial, click here.