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.

India’s regulator has temporarily barred Jane Street from accessing the local securities market, dealing a severe hit to the US firm that allegedly made $4.3 billion in trading gains in the South Asian nation in more than two years.

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.