According to a recent SEC filing, billionaire activist investor Carl Icahn increased his stake in Nuance Communications (NASDAQ:NUAN) from about 11% of common shares outstanding to about 16%.

This comes at a time when Nuance has been struggling with a difficult revenue transition from the way it's being compensated. Many customers have begun updating their purchasing agreements and will no longer pay for services upfront, but instead, only based on usage over the life of the contract.

With Carl Icahn deepening his conviction, some investors may be betting on the chances of a quick turnaround for the company, fueled by activist ambitions. However, as Fool contributor Steve Heller points out, Nuance has implemented a "poison pill" provision, which seriously limits Icahn's (or any activist investor's) ability to maneuver. Check out the video below to get the full story.