To activate this card, your "Karakuri" monster must already be in Defense Position when selected as an attack target. An Attack Position "Karakuri" monster must be successfully attacked one time, and live, before "Karakuri Klock" could be activated, and a face-down "Karakuri" monster won't trigger it at all.
You can protect an Attack Position "Karakuri" monster with "Waboku" and its effect will still switch it to Defense Position, making it possible to use "Karakuri Klock" if it is attacked again.
In effect, this card is more devastating than "Mirror Force" because it destroys ALL face-up monsters your opponent controls, not just those in Attack Position.
You can activate this card if one of your monsters select an opponent's Defense Position "Karakuri" monster as an attack target. (Using cards like "Creature Swap" can help in getting this situation to work).