A player can at any time type an action if he believes that the situation requires this. For example, player types "warn" to let a friendly unit know of an ambush. If no such intervention was planned for in the game, then no result is produced. If the developers did indeed want the player to realise this, then the relevant scripts are in place and the friendly unit changes course to avoid an ambush.
The basic idea is to test the player's initiative and good judgment by requiring him to guess a particular action at set stages during the campaign.