F&O Expiry Volatility: Nifty Swings Sharply as Bank Nifty Drops Over 1,100 Points
India’s benchmark stock indices witnessed an unusually volatile session on September 29 as the monthly futures and options (F&O) expiry coincided with sharp movements during the closing auction session. The Nifty 50 briefly plunged as much as 2.2% before recovering a large part of the sudden decline.
The unusual move was particularly visible in the final phase of trading, highlighting how changes in the derivatives settlement process can amplify short-term price movements around expiry.
Nifty Faces Sharp Swings During Closing Auction
The Nifty 50’s indicative closing level briefly fell to around 22,267 during the closing auction session, representing a decline of roughly 2.2% at the day's most volatile point.
However, the index quickly recovered from the sharp intraday move. It eventually ended at 22,716.2, down 0.28% for the day.
The Sensex also finished lower, declining 0.33% to 72,529.07.
The sharp difference between the temporary indicative level and the final closing level shows just how extreme the volatility became during the closing minutes.
Bank Nifty Also Sees Heavy Volatility
Bank Nifty came under pressure during the session and temporarily slipped below the 53,300 mark during the closing auction.
Despite the sharp movement during the final phase, the index ended at 54,259.95, down about 0.4%.
The temporary fall of more than 1,000 points therefore needs to be viewed in the context of the closing-auction mechanism rather than treated as the index's final daily decline.
Why Did the Market Move So Sharply?
One important factor was the monthly expiry of derivatives contracts and the way the Closing Auction Session (CAS) interacts with derivatives pricing.
The CAS framework was introduced on August 3 for stocks with futures and options contracts. It includes a short auction toward the end of the trading session to help establish closing prices.
On expiry days, however, uncertainty around the final settlement price can have a much larger effect on derivatives positions. Sudden changes in indicative prices can trigger rapid adjustments in futures and options positions, increasing short-term volatility.
The September 29 episode has therefore renewed attention on how the closing auction and derivatives settlement processes interact.
SEBI Is Already Reviewing the Settlement Process
The Securities and Exchange Board of India (SEBI) has proposed changes aimed at reducing extreme price movements around expiry.
One proposal would calculate derivatives settlement prices using trades from the final 30 minutes of regular trading along with the 10-minute closing auction.
Another option would temporarily return to the earlier approach of using only the final 30 minutes of regular trading for settlement, excluding the closing auction for at least a year.
SEBI has also proposed tighter rules around order cancellations and changes to the post-auction derivatives trading period.
Market analysts quoted in the report said measures such as a return to volume-weighted average price (VWAP)-based settlement could potentially reduce uncertainty around expiry-day pricing.
What the Episode Means for Traders
The September 29 movement is a reminder that an index's temporary indicative price during the closing auction may not represent its eventual official closing level.
For traders using futures and options, this distinction is particularly important because settlement prices can directly affect positions and payouts.
It also highlights why unusually large moves during the final minutes of an expiry session should be interpreted carefully. A sudden screen movement does not necessarily mean that the underlying market has genuinely fallen by the same magnitude.
Nifty's Broader Trend Remains Under Pressure
The expiry-day volatility came against a weaker broader market backdrop. The Nifty 50 was down 5.7% during September through the date of the report.
Technical analysts cited in the report identified 22,600 as an important level to watch. A sustained move below that area could increase selling pressure, while holding above it could allow a recovery toward higher levels.
Global factors are also influencing sentiment. Higher US Treasury yields have increased competition for global capital, while elevated crude oil prices remain an important concern for India because the country depends heavily on imported oil.
These factors can influence inflation, the trade balance and corporate profitability, adding to the pressure on Indian equities.
What Investors Should Watch Next
The key takeaway from the September 29 session is that two separate issues need to be considered.
First, the extreme movement during the closing auction was closely connected with the expiry and settlement mechanism. Second, the broader market is already facing pressure from global yields, oil prices, foreign fund flows and technical weakness.
Investors should therefore avoid interpreting the temporary 2.2% Nifty decline during CAS as equivalent to a 2.2% fall in the index's final daily performance.
The final closing figures tell a different story: Nifty ended 0.28% lower, Sensex lost 0.33%, and Bank Nifty declined 0.4%.
For derivatives traders, however, the episode underscores the importance of understanding settlement mechanics and expiry-day risks rather than relying solely on the headline index movement.
