Pre-Open Order Entry And Cancellation Rules

In the minutes before the opening bell, exchanges let traders submit orders that will participate in the opening auction, and they continuously publish an indicative clearing price and imbalance based on what's been entered so far. But if traders could freely enter and cancel orders right up to the last instant, they could flash a large fake order to move the indicative price, then yank it just before the cross — manipulating other participants' reactions without ever intending to trade.

To prevent this, exchanges impose pre-open order entry and cancellation rules: order entry is open early, but as the auction approaches, cancellation rights get progressively restricted — commonly, orders can be freely cancelled up to some cutoff (say, 10 minutes before open), and after that, cancellations are limited to specific reasons (like correcting an erroneous order) rather than allowed at will.

Worked example. A trader enters a 500,000-share buy order 20 minutes before the open, pushing the indicative price up and encouraging other sellers to lower their offers. Under the exchange's rule, once inside the 5-minute pre-open cutoff, that order can no longer be freely cancelled — it either participates in the auction as entered or requires an exchange-approved erroneous-order correction, closing off the "flash and cancel" tactic.