What is the clarification of the condition "that the commanded act be non-existent" in Usul?

The valid conditions for an action subject to a divine command (al-fi'l al-mukallaf bihi) are three: that it be known to the one commanded, that it be non-existent, and that it be possible. The meaning of the action being non-existent is that the prayer and fasting commanded at the time of the request must not be present. Therefore, it is not valid to command something that is already existent and obtained, because to obtain what is already obtained is impossible, as it entails contradiction.