What is Automatic deployment and why does it keeping saying no idle troops?

Automatic deployment doesn't deploy your formations. Instead what it does is assign troops to the formations based on those that you have available and what you have assigned already.

This is useful if you have recently healed troops or have troops spare and your formations are not at their maximum capacity. In this case you will see the cog symbols flashing to let you know that you can add troops to the formations. Instead of opening each formation and adding troops to each wave you can tap the Automatic deployment button to fill any gaps with any idle troops that you have.

If you don't have any spare troops that will fill the spaces available then you will get the message about not having any idle troops.

But my question is how to undo that? if you have accidentally clicked automatic deployment option.