In as much as IB enables options to be exercised instanteously, the writer also has to reckon with midday assignments on that very day, so that if his account size is not big enough, and he is not there to rebalance the position himself, IB will handle matters on their own, which may likely mean dumping the long legs to the spread by day-end (instead of selling the stock and rebalanceing the position for you).
Please verify if this is true.
Please verify if this is true.