Paul Singer's Elliott Management is flexing its activist muscles this week and has plans for much more in 2014.
A portfolio manager at the $23 billion hedge fund firm, Jesse Cohn, is aggressively pushing for changes at technology companies Riverbed Technology and Juniper Networks in a series of public letters meant to prod action—and higher stock prices.
It seems to be to be working.
Juniper jumped from about $22 a share Friday to more than $25 as of Tuesday following a filing from Cohn calling for "cost realignment, capital return to shareholders, and the optimization of Juniper's product portfolio." Elliott, which owns 6.2 percent of Juniper, believes the plan will cause the stock to rise as high as $35 or $40 per share.
"Investors and Street analysts have been calling for Juniper to implement these value-creation initiatives for years, and we believe the three-pronged approach laid out in today's presentation would be very well received," Cohn said.
"Juniper welcomes open communications with its shareholders and values their input," Juniper said in a statement Monday. "The Elliott presentation was received this morning and we have not had any discussions with Elliott with respect to its content. The company intends to review it carefully."
(Read more: After Riverbed, Elliott seeks to shake up Juniper)
Riverbed's stock has also jumped on Elliott's activism. The company's shares were trading around $17.80 but jumped to nearly $20 after Elliott offered to buy Riverbed for $19 a share on Jan. 8. Elliott owns 10.5 percent of the company's stock.
Cohn sent a new letter Tuesday, reiterating the firm's desire for Riverbed to accept its buy-out offer or to shop itself to another party.
"Regardless of the source, the feedback we have received has been overwhelmingly supportive of the idea that Riverbed should fully explore our offer and the other acquisition interest that exists," Cohn wrote in highlighting 10 analyst quotes positive of Elliott's plan. Cohn also warned that the stock price could fall to $14 or $15 a share should Riverbed fail to sell.
(Read more: Riverbed to hedgie Elliott: We'll think about it)
A spokesman for Riverbed declined to comment beyond a statement last week saying the firm was processing the Elliott demands and would eventually respond.
More activism from the hedge fund is likely. Elliott will have a "very active 2014" especially in equity activism, according to a person familiar with firm. A spokesman for the firm declined to comment beyond the letters.
Elliott's flagship hedge fund gained 11.8 percent net of fees in 2013, according to investor documents obtained by CNBC.com.
CNBC reporter Kate Kelly contributed to this story.
—By CNBC's Lawrence Delevingne. Follow him on Twitter