While this card is on the field, Monster Cards cannot be Set or flipped face-down. If a Monster Card would be Set, it must be Normal Summoned in face-up Defense Position.
Rulings
Individual Card FAQs
You can activate (flip face-up) this card at any time; it does not need to be activated in response to a Summon.
The effect of this card applies to both players, and does not affect monsters that are already on the field face-down. These monsters remain face-down until flipped face-up as normal.
While this card is on the field, monsters can be Normal Summoned in face-up Defense Position, and can be the target of "Trap Hole" and will trigger effects like the effect of "Mysterious Puppeteer".
While this card is on the field, if a monster would be Special Summoned in face-down Defense Position (such as with "Cyber Jar" or "Nimble Momonga"), it is instead Special Summoned in face-up Defense Position.
While this card is on the field you cannot flip a monster face-down with "Book of Moon", "Darkness Approaches", etc.
Trap|
Continuous