Before beginning with marching, drilling, or facing it is important to note the difference between a preparatory command and a command of execution. The preparatory of course 'prepares' you for what is to follow. It will tell you to pay attention to execute the command but it will typically also inform you as to what the command is. The command of execution generally is just there to signal when the command begins.
Take for example "Forward March". Forward is the preparatory command. You know that there is only one command that begins that way and you know what do do for Forward March. By the time the command for "March" comes along you already know exactly what to do and you're ready to do it. That second command, the command of execution, only gives you the timing as to when to do it.