India, July 10 -- In April 2024, Jane Street - one of the world's most secretive trading firms - did something uncharacteristic. It went to court. The firm sued rival hedge fund Millennium Management in New York, accusing two former employees of stealing a trading strategy. Not just any strategy. One that, according to Jane Street, had earned it nearly a billion dollars. The focus of the trade? India's Bank Nifty options.

That's where this story begins, say Krishna Jha, a Bengaluru-based investor, and Ashwani Gupta, an Ahmedabad-based futures and options trader.

The lawsuit was settled quietly later that year. But by then, the secret was out. A little-known index with just 12 stocks had become the centrepiece of one of the most lucrativ...