When you are in the middle of an action or attack, and you press the number for your next action (during the period when all actions are greyed), the action gets queued to automatically engage at the end of your current action instead of being denied. This is more for convenience for setting up your next action, rather than spamming a button until it happens. This is not a suggestion to allow for the queue of 5 successive actions, but rather just your next one.