Short selling – the trade where you hope a stock’s value will fall rather than rise. If it does, the trader can win. Big.

That’s led to the growth of what’s called the activist short seller. A trader who comes out to tell the world why a company’s stock should be lower than it is. They’re betting on its failure.

Carson Block from the firm Muddy Waters is one of the most prominent new short sellers – vocal on the news and social media, he explains why the companies he shorts are usually the ones with something to hide.

Producer: Ciaran Tracey