Both the Archer Queen and the Barbarian King  have a special ability from level 5 onwards, which you can activate when attacking.

The Barbarian King's Iron Fist 

The Iron Fist summons up to ten Barbarians ( at the maximum ability level)  and gives the Barbarian King increased damage, speed. He also gets a health boost, unless his health bar is still full.

To activate the Iron Fist during a battle, you first have to deploy your Barbarian King on the battlefield. Then, simply tap again on the Barbarian King Icon to activate the ability.