NSE Nifty up 1.6%, BSE Sensex up 0.7% — same day, same market. What SEBI's new closing rule changed

SEBI's new mechanism replaces a 30-minute average with a single-price auction. The debut sparked confusion. How it works and why it matters.

For most of Monday, the Nifty 50 and the BSE Sensex moved in step. At 3.15pm, they went their separate ways. The Nifty finished 390.70 points, or 1.6%, higher at 24,774.30, while Sensex closed up 544.39 points, or 0.7%, at 78,639.03 — a rare divergence between the two blue-chip indexes.

A man touches the stairs as he arrives at the Bombay Stock Exchange (BSE) building in Mumbai. (Reuters File)

This was no trading glitch, a person familiar with the matter told Reuters. Monday was the first day of the Closing Auction Session (CAS), a mechanism cleared by the Securities and Exchange Board of India (SEBI) that changes how end-of-day prices are set for stocks that also have derivatives trading on them.

Derivatives are financial contracts whose value is tied to the price of an underlying share. They are of two broad categories in India.

A futures contract is an agreement to buy or sell a stock at a fixed price on a set future date. Both sides are bound to complete the trade when that date arrives. An options contract gives the buyer the choice to buy or sell a stock at a fixed price on a set expiry date, in exchange for a fee paid upfront — the buyer can walk away from the trade if the price moves against them, and lose only the fee. Options themselves can also be bought and sold in the market before expiry.

Traders use both to bet on where prices are heading, or to protect existing shareholdings from losses.

What changed on Monday

Until last week, the closing price of these stocks was worked out using the volume-weighted average price, or VWAP, of trades in the final 30 minutes of the day.

As the name suggests, VWAP is the average price of all trades over the half-an-hour period. In this case, the larger trades pull the average more than smaller ones.

That figure would typically become the day's official close and was used to settle derivative contracts, price index funds (mutual funds built to mirror the make-up and returns of a market index such as the Nifty or Sensex) and value passive portfolios (baskets of stocks assembled to track an index rather than picked by a fund manager trying to beat the market), PTI reported.

Under CAS, continuous trading in derivative-linked stocks stops at 3.15pm. Between 3.15pm and 3.35pm, all buy and sell orders for these stocks are collected into a single pool. At the end of that window, the exchange picks one price — the level at which the largest number of shares can actually change hands, where buyers and sellers match up most fully. That price becomes the day's official close.

Derivatives trading itself continues until 3.40pm, and a separate post-close cash session runs from 3.50pm to 4pm, during which pending trades are executed at the newly discovered closing price.

The auction runs within a price band of plus or minus 3% around a reference price, which is the VWAP of trades between 3pm and 3.15pm.

Two kinds of orders can enter the auction. Market orders — instructions to buy or sell at whatever price the auction settles on — get priority over limit orders, which come with a fixed price the trader is willing to accept.

Unexecuted limit orders left over from continuous trading are carried into CAS, though stop-loss orders (which trigger only when a stock hits a pre-set level) and iceberg orders (large orders sliced into smaller visible pieces) are kept out, PTI reported.

CAS applies only to stocks with derivatives contracts. Every other stock in the cash market continues to close on the old VWAP method.

Why the change

SEBI has said the auction is meant to make the closing price more transparent, cut the room for manipulation, and bring India in line with markets abroad, where single-price closing auctions have long been the norm.

According to PTI, the regulator expects the new system to give large institutional orders a cleaner exit, produce fairer settlement of derivatives, and reduce tracking error — the small gap that opens between an index fund's return and the index it is meant to mirror.

Bloomberg noted that India already uses a version of the same idea at the other end of the day, in the 15-minute pre-market auction that sets opening prices and has been in place for over a decade.

Why Monday looked odd

The Nifty's official close ended about 0.8% above where it was trading just before the auction began at 3.15pm, Bloomberg reported. The Sensex, calculated by the BSE, showed no comparable jump — which is where the gap came from.

Because each exchange sets its own closing price from its own order book, the final figures for both depend on how the shares move based on own auctions.

Vaibbhav Sud, founding partner at KKerdos Creators LLP, told Bloomberg that thin trading in the first auction likely exaggerated the move. With a single price now setting the close, even a modest surplus of buy orders over sell orders was enough to push the benchmark sharply higher. Under the earlier method, the same orders would have been spread across 30 minutes of trading, and their effect on the closing level would have been far smaller.

Hariselvan Radhakrishnan, founder and CEO of research firm HST Wealth, told PTI the late move from around 24,589 to about 24,774 came from the new auction structure funnelling orders into a single window and amplifying the end-of-day adjustment, rather than reflecting an intraday trend.

Siddhartha Khemka, head of research for wealth management at Motilal Oswal Financial Services, told Reuters the divergence looked like a single, end-of-session price movement, and that market participants were still waiting for clarification from the NSE.

The NSE, in a late-Monday statement cited by Bloomberg, said the mechanism had worked as designed. The first auction drew 515 trading members placing orders across more than 50,000 unique accounts, and the system would mature over time, the bourse said.

The reaction

Mayank Bansal, a Dubai-based hedge fund trader who deals in Indian derivatives, told Bloomberg the upmove was "essentially fake" and would show up as a gap-down at Tuesday's open.

It largely did. By 10.05am on Tuesday, the Nifty had fallen 0.58% to 24,629.70, while the Sensex added 0.26% to 78,842.70, Reuters reported.

Kranthi Bathini, director of equity strategy at Wealthmills Securities, described Monday's late surge to Reuters as a one-off distortion from the new mechanism that had normalised at the open.

VK Vijayakumar, chief investment strategist at Geojit Investments, told PTI the divergence was a one-day aberration that investors should not read too much into.

The bigger test comes on the weekly derivatives expiry, when open contracts are settled and traders roll positions from one week to the next. "The expiry will be the real test of the new mechanism," Kruti Shah, a quantitative analyst at Equirus Securities, told Bloomberg.

Traders will be watching whether the auction-set close moves in line with futures prices or drifts away from them — a gap that could raise the cost of hedging (the offsetting trades used to protect other positions), complicate settlement and change how traders play the final minutes of the session.

If the auction close keeps landing far from the 3.15pm level, expiry-day volumes could fall sharply, Bansal said.