"Short interest" is a measure of the percentage of a company's outstanding shares that are currently held short. To short a stock means you must first borrow the shares, on margin, and then sell them along with the promise to buy them back at a later date. Until that later date, you pay interest on the shares you have borrowed from someone else. So the bet is that the stock is going down.
There is some helpful information in monitoring the short interest for a stock. If the short interest is increasing it may be an indication that someone(s) has a strong feeling the stock is, or will soon be, in trouble. The mechanics of initiating a large short position also puts a great deal of downside pressure on a stock.
So what happened with Ackman and Herbalife?
You can see that as Ackman's short interest began to climb in 2012 the stock began its sharp decline, getting cut by more than half. But then we can also see that in the beginning of 2013 -- when Carl Icahn, Dan Loeb, George Soros and others came out publicly on the other side of Ackman's trade -- short interest began to come down slightly and the shares climbed out of the cellar. Then short interest plunged almost exactly a year ago and the stock surged to all-time highs. Why did the short interest plunge in late 2013?
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Better cover your shorts!
As the stock price started to rise in early 2013, Ackman's short bet was becoming less and less successful. And more and more costly. Supposedly he sold 20 million shares short, which means every dollar increase in HLF's stock price cost his investors $20 million (in addition to borrowing costs).