ROLL-OVER activity picked up yesterday, boosting relatively thin turnover to about 16,547 contracts. Unlike Wednesday and Thursday, when broker activity appeared to distort the futures, forcing them to big premiums and discounts, April futures tracked the cash yesterday. The contract closed up 75 points at 8,670, a premium over the cash of 25 points. Turnover was 12,829 contracts. May futures closed up 70 points to 8,665 on a turnover of 3,718 contracts. Jardine Fleming said traders mainly concentrated on volatility plays in options. 'Volatility buyers bought May-June 8,000-9,000 strangle time spreads,' a brokerage spokesman said. Sellers disposed of June 9,400 calls against long future positions. May implied volatility fell to 25.5 per cent. The volatility on the May strangles stood around 32 per cent at the close yesterday against 28 per cent in June. In stock futures, brokers said there was some interest in rolling over but that the spreads were too wide. HSBC futures fell 15 cents in April to $91.25, at par with the cash. In Hongkong Telecom the future fell five cents to $15.15, against a close of $15.25 in the cash.