Spell / Trap Cards
Remember that Quick-Play Spell Cards & Trap Cards can be activated during the Battle Phase, but they cannot be activated during the Damage Step unless they specifically modify ATK or DEF ("Reinforcements", "Castle Walls", "Rush Recklessly", "Mirror Wall", "Limiter Removal", etc are OK and can be activated during the Damage Step. "Waboku", "Reverse Trap", "Mirror Force", "Magic Cylinder", "Imperial Order", etc. are not OK and cannot be activated during the Damage Step.)
Counter-Trap cards may be used during the Damage Step to negate an appropriate card. You can use "Magic Jammer" against "Rush Recklessly", for example, or "Seven Tools of the Bandit" against "Reinforcements".
Face-down monsters are not flipped face-up until the Damage Step. If my "Neo the Magic Swordsman" attacks a face-down monster, and it turns out to be "Giant Soldier of Stone", it's too late for me to activate "Waboku" (and prevent damage to my life points). But I can still activate "Reinforcements" to increase "Neo the Magic Swordsman".s ATK in the Damage Step after the monster is flipped. Of course, my opponent could also use "Castle Walls" to raise his monster's DEF.
Flip Effects are not activated until the "Resolve effects" portion of the Damage Step. This is extremely important for effects like "Man-Eater Bug" and "Hane-Hane", as these effects are not activated until after damage calculation is performed. Usually, the "Man-Eater Bug"/"Hane-Hane" is destroyed during damage calculation because their DEF is so weak. However, there is a difference between being destroyed (which happens in "Calculate and apply damage") and being sent to the Graveyard (which happens last, in "Send to the Graveyard"). In-between these 2 portions is when the flip effect is resolved.