At the end of a turn, when you and your opponent deal damage, you also get one energy per lightning symbol rolled. This is tracked below the buttons, next to the rolls and pins, and energy does not disappear (except at the end of a battle). Starting at your next turn, you can use the energy on the abilities, each ability can be executed only once per turn. At the end of the turn, when the damage is done, the ability is locked in, before that time, you can click it again to turn it off and you'll get your energy back.