Yes I agree it is hard to believe - sometimes superstitions don't really make sense although normally they relate to bad luck.
Don't every pull the knife out to help - it may actually be keeping the person alive and stopping a catastrophic haemorrhage.
Lol I picked a horrible example although it does happen. And I'm not calm enough to stay conscious if I ever see a person with a knife sticking out of his body @.@
It is almost a reflexive response to someone who is wounded to pull the knife out - I remember a senior doctor who once did this even though logically he knew he shouldn't have. The patient died because the knife was preventing a catastrophic bleed. Once it was out the patient died before he could be moved to the operating theatre (even though it was one floor away).
Oh that's so sad :| Hopefully, I never have to see anyone with a knife in them.
Well if you do don't pull it out.