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.

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.

Large buy or sell orders—executed seconds before close—nudge prices just enough to swing the outcome of options and futures contracts. It’s subtle. It’s fast.