I have never had the opportunity to try this technique on the street, and I recognise that this is the possible flaw. But if we are talking tactics and training in the gym, it is an area of attack that does deserve some recognition, if only to appreciate that some mad bastard could use it on you and you won't be ready.
The back-cut is the most conventional and accepted style an 'over the top' techniques from the days of Prussian dueling. You could add to this a straight downward strike (snap cut) to the head against those opponents that like to fight 'head first' like a wrestler. Basically, our defensive positions are directed towards the centre/front of our body (Cassidy, Sanchez, McYoung, Janich, Keating), where most damage can be done by the enemy, that makes sense, of course. These over the top strikes are effective because they come in from outside our immediately protected area where we don't expect it. Simple and obvious target areas that we take for granted, and think nothing about, to our detriment. These techniques also take advantage of blind spots, and they are easy and quick to deliver. These two techniques are, however, breaking down methods, softners, non lethal attacks that break down the defences of the enemy, blood the eyes etc. and open the way to a possible lethal attack.
There is only one area that starts to fall outside of this protected area from the groin area to the neck zone and it is a quick kill lethal target area, that is the jugular vein. This is the target area for these techniques. While running seminars around the country, I always include a session sparring with the students, this gives me an opportunity to see what each different martial art has to offer against the knife. Most fall very short of any effective knife techniques in either offence or defence, however, some of the students are very fast and aggressive. Their attacks are launched with gay abandon and are usually very committed. This is where jumping techniques works best.
Another reaction that seems to be instinctive in fighters I have met is to automatically resist directly against force. When I put direct pressure on a limb the immediate reaction is to push against the force, not flow with it. When neutralising the knife hand this subconscious reaction further aids in the success of the technique. The attack also causes an instinctive reaction to protect the side of the head allowing further control of the knife hand.
This technique can be offensive or an immediate counter depending on the flow of the fight. My left hand moves to my opponents knife hand not as a grab but a flat palm trap. The natural reaction is to push against the trap, for a very short time that knife is now neutralised. Controlling the knife hand, and pushing it down also leaves the side of the neck open to attack. Jumping to my left (when fighting a right handed opponent) at 45 degrees into my opponent, my knife hand makes a reverse arch across my opponent's head or neck.
Benefits of the attack are:
It is very hard to defend
It is a moving attack across the body away your opponents other hand;
The opponents knife is momentarily neutralised during the attack and cannot be brought into the fight quickly enough as a defence;
Your momentum carries you back out of danger very quickly; and
The attack is very deep and committed.
Karate students have said to me "we aren't allowed to do that kind of jumping techniques in this style". My reply to them is 'welcome to the real world, do you want to survive or look good'. Like Sun Tsu says, 'attack down hill', the driving force coming down from above really freaks people out and does give you the advantage. I have not seen anyone handle the attack well, the dynamic movement makes it a winner in my book. Try this technique next time you're training, especially when you're working with someone who is not expecting it, and see for yourself. Be careful, it will surprise you how well it works, but you may hurt your partner if you're not careful. It takes a reasonable amount of fitness to complete effectively especially in covering the ground and gaining some height, but it delivers the goods.