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Due to all the edits and clarifications asked I will re write the question tomorrow, thanks for your patience. Leaving this up until then, for reference.

In the setting I'm working on, magitech has created, among other things, a class of weapons called "projection blades".
When such a weapon is activated, it allows the cutting edge of the blade to interact with distant targets as if it were touching them.
Essentially you swing your sword at a training dummy 15 meters away and you cut it just as if you were right in front of it.

More details:

• While the effect itself is invisible, as in, you only see a person swing a sword and something distant getting cut, you can visualize its dynamic by imagining that the effect is projected in a straight line parallel to the blade up to a certain distance (details pending, but for the purpose of this question let's say up to 50 meters, the exact length being adjustable ) and in that area it "bridges" the distance between the edge ofthe blade and whatever object ends in the way, for the purpose of touching and therefore cutting targets.
  This effect doesn't have any other physical properties and therefore can't be used, for example, like a lever.
  It's therefore optimal to trigger the effect at a specific point during a swing, as to avoid interaction with unwanted objects. This is possible thanks to a trigger on the handle.
  To reiterate, the blade itself doesn't physically change in any way.




• The effect moves from the blade to the target almost instantaneously, but this still means that an object in the way of what you want to cut will still partially or completely block your cut. Parrying and deflecting can therefore still be a thing, especially whith another projection blade.




• The effect only transmits force, which is why you have to be moving relative to your target or swing in order to do damage. You can't just point the sword directly at your opponent and pull the trigger to impale him like with some kind of infinite length lightsaber.




• Some energy is lost when you use your weapon this way, meaning you would hit harder with the actual blade rather than with the projected force effect. Obviously the convenience of turning a melee weapon into a ranged weapon at will more than makes up for it.




• Force travels both ways, so your feel your weapon impacting with the target and you can potentially get it stuck, however this is quickly remedied by just turning it off.

Edit for additional details:

• The "projection" can only be sustained for a very little time, details to be decided but let's say it's slightly below an entire second. This is more then enough time to strike and do damage.




• After triggering the effect the magitech device needs to wait a moment before triggering again. Again, details pending but let's say 0.5 seconds. This seems like a small amount of lag but in martial arts and actual fighting I think this would make the weapon less "spammable" and encourage a more precise and deliberate fighting technique.

Given the dynamics and limits described above, what would be a plausible fencing style for users of this kind of weapons? Specifically for duels between projection weapon users.

Edit 2:

• Forgot to mention that in this setting most armor and shields have been made obsolete by other kinds of weapons, creating something like the late 18th century in terms of tactics and equipment. This means that while armor and shields would probably be really effective against projection blades, they simply aren't around anymore and duels take place between unarmored opponents.

Edit 3 with yet more contextualization:

• The weapons are used mainly due to their unique advantage scaling really well with user skill. If you are a good sharpshooter, you are still bound by musket-like weapons that can't fire more than a few shots per minute, but if you are a really good swordsman with a projection blade, you can defeat several unskilled opponents before they can even touch you, and be precise and quick enough that opponents armed with these early guns would actually find themselves at a disadvantage.
  This means thar the weapon is seen on battlefields mostly as a status symbol for officers and decorated veterans, and in civilian use as an implicit statement "I'm using a weapon that requires more skill, beware". So duels happen more due to matters of honour or reputation, like cowboys in the far west and samurai in ancient Japan.
  While there are surely more formalized contests between swordsmen, like academias and the like, the focus of the question should be on duelists of a more spontaneous kind.

Thank you and forgive me if the question is unclear in any way, this is my first post.

Slashing: torque

In the 3rd century before the common era there lived in Sicily a Greek physicist and mathematician named Archimedes; his name literally means Master Mind¹, and he was indeed the greatest physicist and mathematician of the Antiquity.

¹) For the literalists: it actually means Great Planner or Great Thinker.

Among his greatest discoveries was the law of levers; namely, that the torques applied by the effort and the resistance must balance: hence his (attributed) quip that given a fulcrum and a long enough lever he could move the Earth.

Since the effect works as if the blade became very long, and it acts as a lever of the third class, if follows that the sideways force applied by the tip of the blade is minuscule, because the torque applied by the hand of the swords man must equal the torque applied by the resistance of the targets skin and clothes...

Thrusting: aim and tremor

Since slashing won't work, what about thrusting?

Well, here we run in the problem of aiming at the target. The blade is ten meters long, and the target is, say, 50 cm wide; thus the target subtends an angle of about 3 degrees. All right, a good swordsman should be able to aim his blade within 3 degrees. But wait! Not all the target is usefully hit. If the target is wearing any kind of armor then one must hit a specific area on the target, and aiming the tip of the sword at a small area ten meters away becomes a wonder in itself.

Parrying Daggers and Bucklers

Given your responses in the comments, i offer you parrying daggers and bucklers. Compared to a ‘normal’ sized shield, these are far more convinient to use and carry.

Staring with the buckler, as it is the easiest to explain, this is a smaller dinner plate sized shield which is used far more actively than a larger shield. Rather than letting the blade come to you, you meet the blade with your shield. This could be a key defense against a blade which has such a long effective range, allowing you a moment to strike wih your own sword.

Parrying daggers are meant for catching and defecting sword strikes. Against a blade that only exists for about a second, they might not be all that useful as they are intended to bind a sword, allowing you to strike with your own.

However, if possible, the parrying dagger could contain technology similar similar to the projection blades but used in a different way. Rather than emitting a blade of their own, when a projection blade comes into contact with the dagger, rather than that blade only lasting a single second, the blade lasts for 2 or 3 seconds. This gives the user time to trap the projection blade using the dagger before stiking with their own projection blade. To counter this parrying dagger, you would have to only activate your blade after you have passed it, which could be hard to do against a skilled user.

You could also use a swordbreaker which has a similar use to a parrying dagger but uses teeth on the back edge rather than long, trident-like points. If you wanted to use the word differently though, your sword breaker might be the opposite of a parrying dagger. Rather than increasing the length of time the projection blade stays active, any projection blade it comes into contact with is instantly deactivated. A sword breaker would cancel out the effects of a parrying dagger, potentially allowing for some interesting fights if one duellist has a SB and the other has a PD. You could break a bind with a PD using your own SB, for example.

Given the dynamics and limits described above, what would be a plausible fencing style for users of this kind of weapons? Specifically for duels between projection weapon users

One word:

FRENZIED

All sword fighting techniques are based on the idea of closing a gap to strike, then strikes and parries are exchanged until one of the combatants retreats a step or takes a hit.

What your weapons have done is essentially eliminated the option of moving out of striking distance. So once a duel starts thrusts, slashes and parries will be exchanged until one of the combatants drops.

Sword and Shield

I would imagine the best fighting style in this scenario would be using an arming sword (your standard one-handed sword which has a blade about arms length) and some kind of large shield.

Given that this sword’s angle of attack can not be predicted in the same way a traditional sword’s can be, it necessitates the use of a large barrier to block the attack, a shield. The sword itself is inconsequential in this answer, you have already defined what it can do, all i can add is it should be a one-handed blade to allow the use of a shield.

The question then is what type of shield to use? There are several options available though i would say you want a large centre-grip (so it can be held out in front of your) shield that can cover a large area. This leaves us with three choices: the Kite shield, the Tower shield or a large Round shield.

I think you would be best of using a kite shield as it is the lightest of the three (allowing it to be moved and carried more easily) and it would adequately cover your body, even against the timed activation you mentioned. It is a large tear-drop shaped shield with the tip pointing down to your leg. If you wanted to reduce the risk of getting hit from unexpected angles, you may instead want a tower shield which is of a similar size but rectangular.

Given the recent edit has made most of this not relevant i propose an alternative option, the pavise. This is a rectanular shield that freely stands on the ground, traditionally it was used by crossbowmen to give them cover whilst they reloaded. If you are opposed to the use of traditional shields, this might be another option. You would also be able to use a two-handed weapon if that is what you wanted.