Good question, ebrahim! I think riders have tried to find the answer to that for as long as we have competed in jumping. There's been all sorts of ideas, from knocking their legs with steel bars to spikes in the protection (!)boots.
In my mind, I think we are all too geared towards the NICs instead of thinking first and foremost of the PICs.
(And I apologize in advance for the lengthy response!)
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To answer the question, I would say the actual contact hoof/fence is usually the "best" NIC there is since it is immediate and doesn't emanate from the rider/handler.
The "normally" careful horse will try to correct itself after a hit, although there are of course less careful or sensitive horses that aren't bothered by a slap on the hoof.
The main important thing in that case is *NOT* to think: "He hit the fence, I need to find a NIC."
Very few - if any - horses, knock down poles on purpose. If they are of the kind that hardly are aware of their mistake they will probably just become insecure if a NIC is given - which may well lead to more knocked down poles and more NIC and more insecurity, and so on.
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My advice would be to *always* find out out WHY he hit the fence - and work from there - with PIC rather than NIC.
Poor balance (poor canter/trot)? Poor body control (drops a leg)? Coming in wrong to the fence (poor distance control)? Too tense? Too relaxed? Rider or ground crew that is disturbing instead of helping the horse? Pain? Tired or bored? Not enough strength? Inability to handle the height/width/distance between two fences? Simple mistake? Even the most talented horses knocks down "simple" fences every now and then. Is there a particular fence/situation where he knocks them down, or is it every single jump? Or simply a lack of conformation and talent?
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I could go on and on as there are any number of reasons a horse hits a fence, and our job is to find out the cause and fix it by helping the horse with exercises and/or treatments that amends the root of the problem.
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In other words; find the problem, then find an exercise that will help (for example pole exercises to improve the canter/balance/strength/see the distance etc. etc.).
Set him up for success and build his confidence and joy in jumping (lower the bar, poles on the ground to mark where to jump off, build easy distances between fences, get a better rider/ground crew).
Ignore the "mistakes" and give the horse lots of PIC instead when he gets the exercise right.
Remember that fast is slow and slow is fast! Make sure *he* has FUN and eventually you will get a horse that will try his best over the fences. :)

Lovely answer Kicki I think you are SPOT ON! Do keep up the good work, Warmest CI Ann Lindberg

Brilliant Kicki - so right, we focus too much on what we don't want them to do and not enough on what they do right!

thanks first of all
well every one loves to jump and go to competitions and ....but
the challenge is to think and think and think
thanks

Actually, not everyone or every horse does like to jump or go to competitions; this is also an important part of the thinking process :-)