What happened

What a difference one trading day can make for a stock. After rising steeply on Friday, Nordstrom (JWN -1.35%) awoke on Monday with quite the hangover. The retailer's shares tumbled by over 8% on the day, a far worse performance than the S&P 500 index's 0.6% slip. A famous activist investor's involvement in the company was the yo-yo behind the big stock price moves.

So what

Said investor is Ryan Cohen, who rose to prominence as co-founder and onetime CEO of pet products company Chewy before garnering a reputation as an influential activist investor. He's been involved in several high-profile companies, most notably video game retailer GameStop, for which he currently serves as chairman of the board of directors.

Friday morning, citing "people familiar with the matter," The Wall Street Journal reported that Cohen was targeting Nordstrom, amassing a sizable stake in order to agitate for changes to its board. Not surprisingly, given the very bullish turn GameStop took after Cohen became involved in it, Nordstrom's stock rose precipitously on the report.

But, as often happens with sudden news that a determined activist investor is loading up on a company's shares, there was a pullback the following day.

Now what

Much of this can be ascribed to pure profit-taking, as those lucky enough to get in early on the initial rally cash out. Some can also be explained by the morning-after realization that even the sharpest activist investors can take months, even years, to effect meaningful change at one of their targets... assuming they're ultimately successful at all.

So investors would be wise to wait until the dust settles from the Journal's article to gauge how the Cohen/Nordstrom story might play out.