When a face-up Attack Position monster your opponent controls just activated an effect (except during the Damage Step): Negate that effect, and if you do, change that monster to face-up Defense Position.
Just switching your opponent's monster to defense alone won't really help much, but cards like Ninja Grandmaster Sasuke and Tragedy can be used to destroy defense position monsters without needing to target them. If you simply want to stall the duel, you can use Gravity Axe - Grarl to keep them locked, which can be good in farming against certain Legendary Duelists.
Sylvan Komushroomo, Sylvan Komushroomo, Slate Warrior, Jowls of Dark Demise & Magical Merchant
This card can be a good counter to negate Flip Effect monsters such as Sylvan Komushroomo, Slate Warrior, Jowls of Dark Demise or Magical Merchant (situational)
This cards can't stop effects: from hand like Ebola, monster you control like Golem/Parasite, from graveyard like Rose Lover, during damage step like D.D Warrior, monster in defense like most flip effect, monster that sent to graveyard as cost like Exiled Force.
It also only stop the effect once and doesn't destroy the negated monster so cards like Snipe Hunter can spam its effect again. NOPE
I wonder if this card negates Slash Dragon effect. It's still good, negates Hazy, Hammer Shark, Flying Fish, Citadel Whale effects, perhaps also Insect Queen, and Aliens? While Divine Wrath leads to card disadvantage, until your derk works on discard.
Forgot to mention it runing combos of CA. Memory Loss is atm alternative for DL to Fiendish Chain and Breakthrough Skill that would be too game breaking. I can definitely see this card being played. Mark my words.