Knife Fighting Fallacy.

When a dog goes to lay down, it first turns in a circle. Supposedly this behaviour was originally to flatten stiff grass that might have dug into the dog. The dog still makes its circle, even though it is now intending to lie on a soft blanket or your best chair. We mustn't mock the dog, however, since humans are just as prone  to continue with ideas when they are no longer in context, despite our reasoning minds and other advantages
Knife fights are fortunately fairly rare in our culture. Perhaps what is not so fortunate is that when I knife is used it is often in a situation when there is an armament disparity. Either on party is unarmed or faced with a much more potent weapon. This is why no education in self-defence is complete without some knowledge of how to use a knife or how one may be used against you, which is why my book includes quite a long section on knives. As I remark in the book, there is a considerable variety of opinion on how knives should be used, so schools of technique building on unarmed skills, others drawing inspiration from sword fighting. John Styers' book "Cold Steel" very much draws from the knife as a sword tradition. While the book has some very good ideas there is one sequence that stands out as a discrepancy.

Even if we allow for the common practice of distance being exaggerated to make the photo clearer, the idea behind this is not. Why would lowering you knife cause the enemy to lower his guard? Isn't it more likely that dropping your guard would cause an enemy to attack? If Styers is trying to stab the knee or shin for some reason the more likely response is for the enemy to withdraw the leg and counter attack the head or arm, either with his knife or using the leg to riposte with a kick.
This same technique appears in William Cassidy's "The Complete Book of Knife Fighting" where we are told  that lowering your guard will cause your opponent to do the same? Why? With no trace of irony the following picture (fig 66) tells us a low feint will put your opponent's head and arm within reach. In Fig 65 it seems more likely that Randall would naturally flinch back and slash at Loveless' arm.

Styers drew on the techniques of another marine, Col Biddle, author of "Do Or Die". Biddle based his knife fighting techniques on swordplay but the weapons shown used were GI-issue Sword Bayonets with blades of about 17", so techniques such as parrying were more practical. Interestingly, Biddle's book does not show the technique above.
The sequence both Styers and Cassidy show might have actually worked with swords. A sword would be long enough to threaten the leg and parrying it by lowering your own sword would have been a practical response. It might have worked with a machete or sword bayonet. With shorter bowie knives it no longer seems practical and even becomes foolish. Like the dog circling on the smooth floor we see the same actions being used even though the context no longer applies.
What is interesting is that no one involved in the modelling for either Styers' or Cassidy's books actually stopped and said "Hold on, this doesn't make sense!" That perhaps tells us a lot about human nature.