Imbalance Feeds And Indicative Auction Prices

An auction only executes once, at a single moment, but exchanges don't keep the order book that will feed into it secret until then. In the minutes leading up to an opening or closing auction, exchanges broadcast an imbalance feed — a running readout of how lopsided the book currently is, and what price it would clear at if the auction ran right now.

The core numbers published are the paired shares (how many shares are currently matchable between buyers and sellers), the imbalance (how many more shares are on the larger side than can be paired — a net "more buyers than sellers" or vice versa), and the indicative clearing price, the price that would currently maximize matched volume. All three update continuously as new orders arrive or existing ones are cancelled, right up until the auction actually runs.

This feed exists partly to help the auction do its job better: seeing a large imbalance encourages traders on the thin side to step in with new orders, which narrows the imbalance and produces a cleaner, more liquid final match than would happen if nobody could see it coming. But the feed is also watched closely by traders looking to profit directly from the information: a large, persistent buy imbalance heading into the close often pushes the auction price up, so short-term traders sometimes buy in the continuous market beforehand, anticipating the imbalance will lift the closing price further.

Worked example. With ten minutes left before the close, the imbalance feed shows a buy imbalance of 800,000 shares at an indicative price of $120.50, up from $120.00 a few minutes earlier — signaling strong net buying pressure heading into the close, likely from an index fund rebalancing into the stock. A trader watching the feed sees the imbalance growing rather than shrinking as the auction approaches and infers the closing price is likely to print above the current continuous market price, rather than converging back down to it.