What happened

Shares of video- and teleconferencing equipment maker Plantronics (NYSE:PLT), also known as "Poly" since its merger with Polycom, are plunging Friday, down 17% as of 10:45 a.m. EDT on news that one of its largest shareholders has just become a much smaller shareholder.  

So what

Yesterday after close of trading, Plantronics revealed that Siris Capital Group, which previously owned 7.1 million shares of Plantronics, has sold 4.1 million of those shares, or about 57% of its stake (and 10% of Plantronics' total share count).  

In its statement, Plantronics explained that Siris' move is simply designed to capture "what it considers to be an attractive return on its original investment in Polycom in 2016." Indeed, Siris Managing Partner Frank Baker reiterated his "confidence in Poly, its management team, and the opportunities ahead."

Wilting sunflower

Image source: Getty Images.

Now what

Of course, with Siris still owning 3 million shares of Plantronics, a $36 million investment, that's what you'd expect Siris to say -- to preserve the value of its remaining investment in the company. Other shareholders, however, are probably wondering what all the sudden selling by such a large shareholder is going to do to the value of their own shares.

So far today, the answer appears to be that it will cost them 17% of that value.

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