This is from military basic training. It takes two commands to initiate something on parade:
- 1) The preparatory command
- 2) The command of execution
The preparatory command is the drill sergeant telling everybody what is coming next. The command of execution is the drill sergeant telling everyone to start doing it. When you have inexperienced new recruits, some will start to execute on the preparatory command, and the drill sergeant cautions the recruit, "Wait for it!"
This is lampooned often in British film and TV: