Well, more kudos for you, if you decided the hold by instinct alone.



To answer your question:

First of all i use "you" instead of "Someone" because i'm not very confident in my english.

Then, about the question:

There are several ways to hold a knife or any weapon the wrong way. The point in using a weapon to fight or even to intimidate is the same as fighting bare-handed.



1) incapacitate or kill the opponent.

2) walk away from the fight.



For sure the probability that a knife-stab connects even from a wrong movement is higher than with a kick or a punch.

And usually who gets affected by a poorly thrown punch or kick is affected for psychological reasons rather than real damage or neurologic shut-down.

But to put down an experienced fighter one must cause real damage to the limbs or to the brain.



Even more important, the damage must be immediate and decisive. If you cut an arteria on a leg the opponent will die if doesn't get medical attention quickly, but this doesn't mean he can't kill you anymore.



A scratch to the cheekbone or a bleeding lip would stop most of the average law abiding persons, but against a fighter it only means you end in a disadvantageous position for at least a second against someone who needs just a fraction of second to take advantage of it.

you can't cause more of a graze, if you can't move your humerus in the correct direction, and if average joe (and probably myself) would be too scared to try and counter in that moment, an experienced fighter won't miss the chance. A cut in the face means nothing to them, and they are pretty sure they won't get even that.



Let's put this way:



If the average person is running away from somebody finds a loaded gun on a table and decides to shoot the pursuer from 10 m away, would miss them, and probably will dislocate some of own joints.

If the average person is running away from a trainded fencer, finds a sword the exact same model and decides to turn and defend from the fencer, is dead the very moment the pursuer is within striking distance.