When are checks done to see if something dies, and does this happen during a showdown before it ends?

Ruling: Currently, cleanup (when death is checked) happens anytime something resolves, a unit moves, the turn ends, and showdown ends. This means in the Yasuo/Volibear example, Volibear would die during the showdown after Last Breath resolves, before Yasuo would die.

Sequence:

• Yasuo deals 6 damage to 9 might Volibear during showdown
• Last Breath is played, dealing another 6 damage to Volibear
• Cleanup happens when Last Breath resolves
• Volibear dies (12 total damage)
• Yasuo survives

Nuances:

• This rule is changing in the new Comprehensive Rules, where cleanup will happen any time the game state changes
• Healing happens at the end of Combat and end of Turn, not during cleanup
• All combats are showdowns