There's been an 'unheard of' stock market shift this week and it's crushing hedge funds. Here's everything you need to know.

Paulo Whitaker/Reuters

To casual observers, the stock market has traded in a relatively benign fashion over the past few days.

But some remarkable shifts that took place underneath the surface caught the attention of strategists, and are inflicting severe pain on a number of hedge funds.

In short, there’s been a massive rotation away from the best-performing stocks and into those that had been neglected. That is known as a rotation from momentum stocks, or those that have had the wind behind their backs, to value stocks, or those stocks that had been ignored and are considered cheaper.

Goldman Sachs said in a note that the decline on momentum “ranks among the sharpest on record,” and Morgan Stanley sent a memo to hedge-fund clients, seen by Business Insider, trying to explain the moves and saying the pain could well keep going.

Here’s what you need to know:

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