The Darn Jinn is a double edged straight sword. The length of the blade should be from the wrist to the top of the ear. Their are many variations of this weapon. Each style has its own version of the darn jinn and many styles have different variations that they use. The variations range from a thick, relatively non-flexible blade from, i think, 7 star preying mantis to a belt sword. The belt sword is so flexible that when pointed upward the blade will hang over your hand. It is considered advanced due to control issues and because you need some control over energy to make it rigid enough to block other weapons. There is also a two handed straight sword and a "dragon fast" sword which is slightly shorter than the two handed but can be used with one or two hands. I won't even try to go into the European weapon variations.
The Darn Jinn of any variation was considered a master weapon. It was also carried by the rich and as a badge of office but I won't go into them. It was carried by generals, who( if they lived) were masters. It is felt that the Darn Jinn uses techniques from all weapons and therefore all other weapons are basic training for the Darn Jinn. It is also considered a balanced weapon. By balanced I mean the five elements. This is usually explained as 1. a metal blade, 2. wood handle, 3. Carbon was added to the metal for strength(earth), 4. Fire was used through out the entire process, and 5. the flexible blade moves like water(it was also quenched in water). The more esoteric explanation tends to follow this path- 1. the techniques are fast like fire, 2. the footwork must be fluid like water, 3. the mind must be centered like earth, 4. The yi (intention) must be focused to a piercing intensity like metal, and 5. the body must be supple like the bamboo (wood). This is not a standard eplanation but I have heard and read of quite a few and these are the most extreme I have heard. Of course the five elements are in every weapon if you look hard enough but I like to have things like this ready in case someone askes.
The Darn Jinn training mainly focuses on precision techniques. The main targets are tendons and arteries, fingers, lips, nose, ears, any piercing, and the eyes. You can hack and stab with it but people do that lip curl, like when you put catsup on spare rib. Traditionally the Darn Jinn was as sharp as possible for the top third, sharp enough in the middle and dull for the bottom third. This was because if you block with a sharpened blade it will chip and eventually break the sword, but if dull the sword life would be extended. The middle was not that sharp because it was used to chisle or scrape the muscle from the bone and you didn't want to spend your whole life resharpening the blade. The tip was almost razor sharp because you didn't want the opponent to feel the wound till after it was too late. Being flexible also allowed the sword to find weak points in armour easier which made it ideal for generals and in the city.