Dexin-B (02526) advanced 21.1% to HK$600, on turnover of about HK$17.73 million from 30,000 shares, according to ETNet.

The stock peaked at HK$600 and traded as low as HK$465 during the session.

At 9:49 AM, the surge triggered the exchange’s volatility-control mechanism, sending the stock into a five-minute cooling-off period during which trades were restricted to between HK$446.4 and HK$603.5. Normal trading was scheduled to resume at 9:54 AM.

Dexin-B is a constituent of the Hang Seng Composite MidCap Index. The cooling-off rule activates when a stock’s price moves more than 15% from the last traded price five minutes earlier in continuous trading sessions.