TRADING activity in Hang Seng Index futures fell to more normal levels at 21,410 contracts traded, after reaching a record on Wednesday of more than 50,000 contracts. The April contract expired at 8,312, 428 points off its high of 8,740. In trading, the contract rose 21 points to 8,311. Turnover was 4,005. In May futures, there was a turnover of 17,390 contracts. The contract closed up 40 points at 8,345, a premium over the cash of 33 points. Some dealers suggested the increased volatility from April 18 onwards on a falling cash market had saved quite a number of investors from substantial losses in short positions. Open interest for Wednesday in April was 11,486 contracts. In May it was 31,683 contracts. Jardine Fleming said in index option trading: 'Premium sellers dominated, with interest focused on May puts.' The brokerage said overseas bulls sold May 7,800-8,200 risk reversals. The front-month implied volatility fell to 26.25 per cent. A risk reversal is where a pair of options are combined to provide a pay-off pattern in an equity instrument similar to the interest rate collar or range forward contract in fixed income or currency markets respectively, says the Swiss Bank Corp Dictionary of Financial Risk Management.