When Ava attacks and you use her ability to play a card from hand, when do you pay the Mind cost and when do you reveal the card?

Ruling: When Ava attacks and triggers her ability, you do not pay the Mind cost until the ability resolves. You keep the card hidden in your hand until resolution, then pay the Mind cost (if you choose to) and play the card. The Mind cost is part of the effect, not a cost to activate the ability.

Sequence:

• Declare Ava's attack and her triggered ability
• Keep the card you intend to play hidden in your hand
• When the ability resolves, you may pay the Mind cost
• If you pay, reveal and play the card from hand

Nuances:

• Ava must remain on the battlefield for the effect to resolve (the ability says "here")
• If Ava is bounced to hand (like with Gust), the effect does not resolve and you keep the card in hand
• If Ava is moved to base, the ability can still resolve since she is still "here"
• Triggered abilities do not have costs - the "you may pay" part happens after the comma in the condition, making it part of the effect
• This works like Bullet Time, not like Get Excited