THIN volume in stocks meant Hang Seng Index futures were able to drag, or push, the cash market around all day yesterday. In the morning, aggressive overseas buying saw the February contract reach the high of the day near the opening at 8,020 points. Selling pressure at 8,000 saw the contract slide to the low of the day of 7,870. The February future was at discount all morning followed by periods of running to a premium in the afternoon. That helped push the market up. Overall market activity was heavy, buoyed by roll-over activity, with 33,893 contracts traded. The February contract closed 150 points up at 7,995 . In index options, Jardine Fleming said: 'Premium buyers continued to dominate with attention switching to March options.' The brokerage said traders bought March 8,000 straddles and 7,800-8,200 strangles. Implied volatility rose. A buy straddle is where an investor buys a put and a call at-the-money in the belief they will make money from a dramatic move in the underlying, irrespective of direction. A buy strangle is where investors buy out-of-the-money puts and calls on a similar basis to the straddle buyers, but it has a more aggressive risk profile.