Why is it that the swordsman's attacks appear to do nothing until they click the sword back into its sheath? For example, in One Piece, Brook, and sometimes Zorro, do some big attack with their swords, but the cuts are not shown until after they return their swords to the sheaths.
This trope is usually attributed to Fist of The North Star, after Kenshiro attacks someone he pauses and says "You are already dead". It is only then that they realise their defeat and the attacks are applied.



Reference. 
Another Reference (TVTropes)

It's similar to how in Road Runner, Wilee Coyotee often runs off of cliffs, without realising the ground below him has stopped - he only falls when this is pointed out to him.



The usual rationale is that everything happens so fast that the character cannot register what is going on - and neither does their body.