HANG Seng Index futures rallied a second day yesterday in modest turnover as volatilities in index options came off in bear trading. Overseas and Hong Kong buying helped the May future, which opened at 8,550 points, on the back of strong buying of Hong Kong stocks in London, and closed 135 points up at 8,620. The cash market closed up 207.39 points at 8,620.27. Fimat Futures said: ''The market was unable to break the 8,700 resistance.'' The June future was up 145 points to 8,620. Overall market turnover was 13,231 contracts with 13,176 in May and 30 in June. Open interest in May was down to 32,867 contracts and 417 in June. In index options, Jardine Fleming said: ''Bearish investors were active in the options market. ''Local investors sold the May 8,600 and the 8,800 calls heavily.'' Some overseas investors bought May 9,000-8,600 put spreads. Under the pressure of selling, implied volatility dropped to around 34.5 per cent in the front month. Hang Seng Index 100-day historic volatility was at 40 per cent. Option volume was active at 2,766 lots and the open interest was 29,670.