India has barred one of the world's largest quant trading firms, Jane Street, from accessing its securities market after an investigation found it made "unlawful gains", taking the most stringent action ever against a foreign trading firm.

The lawsuit alleged that the traders had stolen a “ highly valuable, unique, and proprietary ” trading strategy, which was quickly revealed to revolve around India’s booming options market. Jane Street and Millennium settled the lawsuit in December, but judging from SEBI’s report it has cast a long (and expensive) shadow.

At the heart of the controversy is Jane Street’s aggressive trading in the derivatives (futures) segment, where the firm executed trades designed not just to profit, but to sway the market itself. Here's how.