You can upgrade your hero's skills to increase skill attack stats. Every time you promote a hero, a new skill is unlocked. Tapping on the skills' pictures allows you see details about each hero’s unique set of skills. For every hero, there is one manually casted skill (based on energy), and three auto casted skills. During battle, once your hero reaches full energy, you will see the yellow light box, tap this box and tap the enemy you want to attack in order to unleash the manual skill.