Spell Proof Armor (STP4-ENS03) - Speed Duel: Tournament Pack 4 Unlimited

Activate this Skill during your Main Phase and apply the following:
• Machine Normal Monsters you control are unaffected by your opponent's Spell Cards or effects.
• While you control no monsters, you can Normal Summon Machine Normal Monsters with 1 less Tribute.
• If you have a non-Machine monster on your field or GY, flip this card over. This Skill can only be used once per Duel.