Why does Zenith Blade allow a yellow Leona to trigger her 'When I attack' ability when played during the Focus window after attackers are already declared?

Ruling: When I attack/defend triggers now happen whenever a unit becomes an attacker or defender, not just during the initial declaration phase. When Zenith Blade moves yellow Leona to the battlefield during the Focus window, she becomes an attacker and her 'When I attack' ability triggers.

Nuances:

• Enemy units that were already declared as defenders earlier do not trigger their 'When I defend' abilities again when a new attacker joins
• The Teemo section in the FAQ provides additional clarification on this timing rule