During the attack declaration window, due to Fast Effect Timing, the turn player must first activate any mandatory effects that they triggered (such as those of "Fluffal Leo", "Reverse Breaker", etc.), then their opponent must do the same ("Vengeful Shinobi", "Ordeal of a Traveler", etc.). Then, the turn player has the opportunity to activate optional effects like "Ego Boost", before the opponent has the opportunity to do the same. After this Chain resolves, or if neither player activated any cards or effects in response to the attack declaration, then the attack declaration window is closed, and cards cannot be activated in response to the attack declaration for that attack. Players may still continue to activate cards and effects in the Battle Step, so long as their activation requirements aren't dependent on an attack declaration.