Finance Minister Nirmala Sitharaman’s announcement proposing a hike in the Securities Transaction Tax (STT) on Futures to 0.05 per cent from the current 0.02 per cent sent the stock markets into a tailspin, with Nifty50 and BSE Sensex falling nearly 2 per cent in the special trading session on Sunday. Investors became poorer by about Rs 9.40 lakh crore amid sharp sell-off.Nifty50 ended the session at 24,825.45, down 495.20 points or 1.96 per cent. During the day, it tumbled 748.9 points or 2.95 per cent to 24,571.75.The BSE Sensex fell sharply by 2,370.36 points or 2.88 per cent to slide below the 80,000-mark at 79,899.42 in the afternoon trade. It settled at 80,722.94, down 1,546.84 points or 1.88 per cent.Sun Pharmaceutical Industries was the biggest gainer on the Sensex after the Budget speech, climbing 0.77 per cent, followed by Tata Consultancy Services, which gained 0.71 per cent. HDFC Bank showed no change and remained flat at 0.00 per cent.State Bank of India lost 5.61 per cent and Adani Ports 5.53 per cent. Bharat Electronics, ITC, Tata Steel, UltraTech Cement and Reliance Industries were also among the laggards.In the stock market, F&O trading refers to Futures & Options. A Futures is a commitment to buy or sell an asset at a set price on a future date. An Options gives the trader the right, but not the obligation, to trade the asset at a specific price. These instruments allow for high leverage, meaning traders can control large positions with relatively small capital, but they carry significant risk.The STT is a government tax applied to market transactions. The hike directly raises the cost for traders, especially those who trade frequently or in large volumes.Sitharaman’s announcement makes Futures trades slightly more expensive. An STT Options hike means the government has increased the securities transaction tax charged on Options trades, which raises the cost of buying or selling options in the stock market.