Execution is atomic: if there is a $1m 'sell' posted and you issue a $1m 'buy' against it, then either that completes or is rejected (e.g. if someone else has matched it in the meantime).

The point of this system is to send N orders to N exchanges in a simultaneous enough way that an HFT trader can't spot an order executing on exchange A and then issue their own order against exchange B.