What: Shares of Mattel (NASDAQ:MAT) rose as much as 9.6% early Tuesday, then settled to trade up around 5.7% as of 1:30 p.m. after activist hedge fund Jana Partners revealed a new stake in the toy company.

So what: According to regulatory filings, Jana Partners now owns just over 9.31 million shares of Mattel -- or roughly 2.74% of Mattel's total shares outstanding -- worth nearly $232.8 million as of this writing.

Jana Partners is known for amassing large stakes in companies with the aim of pressuring them to implement changes and create shareholder value. Earlier this year, for example, Jana Partners built a $1.2 billion stake in packaged food giant ConAgra Foods (NYSE:CAG), then subsequently convinced the company to expand its board by two selected members to 14. Shortly after Jana's initial stake was revealed, ConAgra also announced plans to sell its ailing private label operations to TreeHouse Foods for $2.7 billion in cash.

Now what: Mattel, for its part, has struggled to find growth as it attempts to modernize its largely traditional toy lines, which have steadily become less appealing to kids in today's digital world. Worldwide sales fell 11% last quarter, hurt by broad-based declines across each of its Mattel, Fischer-Price, American Girl, and Construction Arts & Crafts brands. Operating income and adjusted net income per share both fell a more harrowing 27%. Though to its credit, Mattel is also working to improve retail execution, reduce costs, and build scale in emerging markets, and those results were largely inline with management's expectations at this point in the company's planned turnaround.

Investors should be open to the possibility that Jana Partners' stake is passive -- it may have no plans to weigh in with its own demands of Mattel. But with shares of Mattel still down nearly 20% so far in 2015, it's hard to blame the market for hoping Jana may spur more decisive action to create shareholder value in the near future.

Steve Symington has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.