There are actually 6 positions (I shy away form the word stances because to me they represent "still photo training" and combat is balance and power in motion). In short, I mean that there is no there is no..."football scoring stance." Fighting is balance and power in motion.
Fighting stances are to
a) photo shoots
b) get your class lined up, organized and ready
b) scare opponents
c) maybe get your mind ready and on straight for a confrontation
d) then it becomes balance and power in motion
In the big picture of Knife fight postions there are:
1) Standing/Moving - knife up front front
2) Standing/Moving - knife neutral (somehwer near the hip)
3) Standing/Moving - knife back
4) Knee high (or you can work on seated here)
5) On your back
6) On your sides
Any knife course MUST include training in all these positions or else it is simply not complete for the real world. And you will probably transitions through many positions in just one single fight.
Many militaries of the world have a knife back stance with the empty hand up front for cover. The US Marines, once knife forward, then by Quantico committee in 1999? 2000? (I was around there then and knew why and when it happened) took this stance from front to mandatory rear, to the dismay of many vets on the committee.
The empty and up front cover hand has been called everything in the military like "the cover arm," "the protecting arm (protecting the knife that is) and possibly the worst, "the sacrifice arm."
a) When two people knife spar and one is knife back and one knife forward, the knife forward person has many tactical advantages.
b) When two people knife spar and both have knife back they are equal players in that sense.
c) when a knifer is confronting an unarmed man or a recently disarmed man , a knife back stance can have advantages as the empty hand can block, strike and grapple and so forth.
d) The empty hand should grab something nearby (I love this phrase "exploiting the environment" ) and use it in his so-called duel. In which case this "shield" would be used like an old warrior, knife back. Shield forward.
Like to break more wind on this, but I its time to eat breakfast!
Hock