Whenever you cast a spell, Mindmoil’s ability triggers. The ability will resolve before the spell does.

After Mindmoil’s ability triggers, players may cast instants and activate activated abilities. Each time you cast an instant spell “in response,” Mindmoil’s ability triggers again.

After Mindmoil’s ability resolves, but before the spell that triggered it does, you can cast instants from your new hand of cards. Each time you do, Mindmoil’s ability triggers again.