What you were expecting to happen, and what actually happened:
Expected 20% of enemy extra turns to be denied, so went to Goblin PvP to purposely let them loop and see the effect at work. Not a single time in half an hour of testing did I notice the effect trigger.
How often does this happen? When did it begin happening?
All the time since acquiring Time Warp
Steps to make it happen again
Acquire Time Warp, and watch nothing happen.
I just bought this stone;
Went to underspre dayli and instantly got enemy’s 4x match rejected extra turn (no text appeared as well). So it works, maybe some other conditions