Sensex swings wildly during first Closing Auction Session on expiry day

SEBI rolled out the Closing Auction Session (CAS) on August 3 to determine closing prices for roughly 200 stocks used in equity derivatives, aiming to curb price manipulation. On the first monthly expiry after its launch, BSE’s Sensex experienced dramatic turbulence: it plunged more than 2,200 points between 3:18 pm and 3:23 pm, then reclaimed almost 2,000 points in the following seven minutes, ultimately closing 539 points below the prior day’s level.

NSE’s Nifty showed a milder dip of over 100 points before ending 117 points lower. Leading Sensex components—including Reliance Industries, which fell from Rs 1,289 to Rs 1,250 and rebounded to Rs 1,286, as well as HDFC Bank, ITC and Bharti Airtel—mirrored the swings. Traders voiced outrage on X, noting that buying a 76,500 put option at 3:15 pm and exiting at 3:20 pm could have generated a Rs 44 lakh profit. The episode follows SEBI’s recent bans on Copthall Mauritius, a JP Morgan arm, and domestic broker Mansi Stock & Share Broking for manipulating the Sensex and extracting illegal gains of nearly Rs 3 crore.