GROWING expectations of a strong autumn rally in Hong Kong blue chip stocks triggered a 242-point leap in the Hang Seng Index to 9,929. Turnover was strong at $6.49 billion, up about $1.5 billion on Tuesday's total. Tuesday's 200-point leap was expected and explained by futures-linked trading as a number of dealers with large positions were keen to see the August contract close on a high. Yesterday's leap took many investors by surprise and short covering was a significant feature of trading in the day. A combination of factors have helped the market to head off on a mini-surge. The perception that interest rates are to remain settled for the medium term has helped sentiment towards equities. Morgan Grenfell Asia director David Lavington said: ''This has fuelled buying on Wall Street which, in turn, fuelled buying here.'' Another factor pushing up the market was the reappraisal of the property developers in the light of the disappointing land auction last week and the better than expected interims from Cheung Kong last Thursday. Some investors overseas are nervous they are in danger of missing out on the traditional autumn rally leading into the Chinese New Year rally. During each autumn for the last four years, investors have witnessed a major rally in the index. It has at times been broken by events, including Beijing's attacks on Hong Kong Governor Chris Patten in 1992, but the overall pattern of movement was up. Furthermore derivative-linked traders have plotted a similar pattern in the autumn period, when traditionally implied volatility has risen significantly from the summer slumber of around 26 per cent to the autumn frenzy of, at times, above 50 per cent. Jardine Fleming said there had been a significant switch in derivative-linked dealing from selling premium, with a view to gaining income on the basis that the index was going nowhere, to buying premium, in the belief that volatility was going to go up. Hang Seng Index sub-sector property stocks led the way as the re-rating of the sector continued amid improved sentiment. Properties was up 3.69 per cent to 17,076.47. The property component of commerce and industry pushed this sub-index up. It rose 2.74 per cent to 8,058.46. Finance was up 1.83 per cent and utilities were up 1.3 per cent. Leading trading by turnover was HSBC, up $1.75 to $91 on turnover of $505.78 million. A key gainer was Cheung Kong, up $1 to $39.10, on turnover of $464.77 million, the second biggest of the day. Stablemate Hutchison Whampoa was the fourth most heavily traded stock, rising $1.10 to $38.70 on turnover of $352 million. Sun Hung Kai Properties rose $2 to $57 on turnover of $354.6 million. Hongkong Land rose $1.05 to $20.45 on turnover of $243.8 million. Hongkong Telecom hit a record close of $17, up 10 cents on $217.5 million. A significant feature of trading has been the explosion in share price movement and volume at Jardine Strategic and Jardine Matheson, both of which are moving their trading to Singapore. Dealers are saying the stocks are being re-rated to Singapore valuations. This year, Jardine Matheson outperformed the index by 21 per cent, rising in price terms 39.9 per cent. In the last three or four trading days, the stock has risen 10 per cent and yesterday it was up $4.75 to $72.75 on turnover of $136 million. Jardine Strategic had an even more spectacular day. For an investment holding company it did exceptionally well, rising $2.90 to $32 on a turnover of $116 million, in unusual price movement, trading and volume.