If a unit is moved into a showdown after the single combat phase, does it trigger its 'When I Attack' effect?

Ruling: Yes, 'When I Attack' triggers activate when a unit gains the 'attacker' designation, which happens when they move into a battlefield controlled by an opponent or become attackers through movement spells/abilities during combat.

Nuances:

• The trigger only activates the first time a unit gains the attacker designation in a combat - you cannot move a unit in and out repeatedly to trigger it multiple times.