Comsec works basically like this as I understand. Let's say you have $10k in your account. They let you buy $20k in stocks. You buy $20k in a stock and then sell the same day. Despite the fact that it looks like you are back to even on your account because you bought and sold. The buy transaction is not 'settled' for two days. Thus tomorrow, you cannot buy any more stock even though you have $10k in your account.
