Why does Falling Star kill a Poro on the battlefield when Master Yi's leader ability should make it 4 might?

Ruling: Master Yi does not boost the Poro because attackers and defenders are only designated once a combat showdown starts, and a showdown cannot start while there is an item on the chain. Since Falling Star is on the chain when it resolves, the Poro is not yet a defender and does not receive Master Yi's bonus.

Sequence:

• Falling Star is played and goes on the chain
• Falling Star resolves while still on the chain
• The Poro is not yet designated as a defender because the showdown has not started
• Master Yi's ability does not apply because the Poro is not defending
• Falling Star kills the Poro at its base might

Nuances:

• The player cannot move a unit to the battlefield as a reaction to gain the Master Yi bonus, because showdowns and combats require empty chains
• Units only become attackers or defenders when a combat showdown actually starts, not when the battlefield becomes contested