What the 🔥, I played against 40 lvl Mai and she attacked my facedown snowman eater with amazoness chain master that was special summoned with the effect of my dynatherium - he flips, she receives 400 damage and then I DON'T get to pop a card, she just ends her turn like that's normal. What the 🔥!?
Depends on when it activates, if it was flipped face-up because of an attack, then the effect activates during Damage Step, so of course you can't use Mask Change, you can't summon a monster during Damage Step.
I remember this card being used a lot in 2010 with e-heros for absolute zero.