What's it for? Isn't it kinda useless? Where's the application in it?
Well, here's my take, and obviously highly influenced by my time training with Sonny, whose Carenza was indeed a thing of beauty.
Firstly, as a personal practice it is an awesome way of gaining skills manipulating the weapon, learning how it moves, and making it flow as part of the body.
It can also be a way of familiarizing oneself with a particular blade design, where it's power lines are, how it recycles between strikes, how it thrusts and slices.
But are there uses past that? Can there skills be of use with an opponent present? Isn't it just fancy and unnecessary? Perhaps even a dangerous waste of time?
First off, here is a quote from Lt Col W E Fairburn, of Fairburn Sykes fame:
"I believe that a knife should be bright and highly polished for the reason that 20% of the fight is lost by not striking awe in the mind of the victim that a flashing knife gives."
The psychological effect of an edged weapon is part of it's character, and thus should not be dismissed in training. One of Sonny's questions to me when we worked was: "Do you want your opponent to see your blade? Or not see it?" Truly, both are important parts of the whole.
Sometimes not being seen is crucial, but then so is being seen when they can already see you and are waiting for you to act. Non telegraphic and fully committed motion definitely has a place here, but if you are not fast enough, or are behind the curve to start with, then visual and psychological deception are your friend.
Sonny avoided altercations on more than one occasion by flourishing his blade and having his opponents think better of engaging. He also used his Carenza to confuse and disorient his opponents, thus creating time in the OODA loop to do what he needed to do. Remember time and space are interchangeable, and thus movement is key to this being successful.
This lack of movement, or using the Carenza in the wrong context (like standing still at long range against a gun) gives this deceptive element a bad rep.
But used in the correct place, as a way to close distance and open up a guarded line, it is very useful indeed. In fact I would say that it is the best way, combined with footwork to move off line and change angle, to get away from the glorious double death outcome so often seen.
I think one major reason why people don't respect the utility of this part of training edged weapons is the multitude of 'twirling' videos on youtube that have given it a bad name. I've actually written about this subject before if you are interested: http://swordandcircle.blogspot.com/2012/08/carenza-rant.html
One last thing - perhaps the context of the usage can be widened to think about any situation where escape and evasion are paramount. Think of juking and the Malicia of Capoeira as examples without weapons?
Obviously the utility also holds true for knives in the 'real world'.
Here is Mark Human of Multi Dimensional Warriors in South Africa talking about his perspective:
"We make a point of ensuring that we stay up to date with how violence transpires/ particularly related to edged weapons attacks and confrontations. Research includes interviews with victims, practically working in the field, reading accounts and watching footage of fights and stabbings. There are knife ambushes and attacks and yes there are such things as knife fights and even ambushes that evolve into knife fights that resemble duels. Interactions include hard and fast attacks, fast and flowing attacks, committed and non committed lunges, fakes and picks, unimaginative single strikes or complex combinations and really any combination of anything the human mind can imagine. Don't box yourself into finite presumptions of how someone will attack- to improve your chance of victory become a navigator of chaos by understanding the framework of chaos."
Remember, like Sonny said (paraphrasing here), there is no art in killing, and none in dying, but living ... that's where the art is ...