IN a volatile day of trading, Hang Seng Index futures swung through 175 points as bears gripped sentiment. December futures put on 15 points to close at 8,380, falling to a discount on cash of 32 points. The January contract ended the day 35 points down at 8,360. Overall market activity of 33,228 contracts was boosted by rollover activity, with 19,837 traded in December and 13,389 in January. Index options saw 2,235 lots traded. At-the-money implied volatility in calls and puts in December stood at 23 and 24 per cent. Because of the heavy time decay on the month, dealers are taking their benchmark volatility number from January. The same measure taken in January was 32 and 29 per cent respectively. In March, it was 29 and 30 per cent respectively, which was the same as for June. Open interest for Wednesday in futures was 29,504 contracts in December, 14,251 in January and 388 in March. In index calls and puts, the figures were respectively 18,955 lots and 16,522 lots in December, 1,307 and 3,148 in January, and 4,008 and 2,883 in March. Trading is forecast to quieten down over the Christmas period. Sentiment is still expected to be dominated by developments on Wall Street. Brokers are anticipating some increased activity in January as underweight investors put their portfolios on a more normal footing, having reported their December 31 holdings.