This is a basic, but critical question. I've noticed some variation among practitioners.
- What is the proper grip for Chinese straight sword?
I'm going to use Major Methods of Wudang Sword as the reference, as the information comes from the last generation of Chinese fighters who used swords in combat and duels. In a sense, everything after this generation is somewhat academic, and purely in the realm of sport and performing art.
In the chapter Shou Fa (Hand Techniques) The book describes two grips:
"Normally people hold the sword very firmly which is called Si Ba Jian (Dead-grip Sword). The advantage is that it is not easy for the sword to be knocked from your hand."
This is the way one is taught at the beginning, typically in regard to "external" sword form. Here a teacher would simply tell the student to hold the sword in their fist, and not change that grip. However, the book notes that "The disadvantage to [dead-grip sword] is that you cannot use the sword in a flexible manner." This leads to the more advanced method:
"In Wudang Sword, you hold the sword loosely, which is called Huo Ba Jian (Live-grip Sword). The good thing about Hua Ba Jian is that you can manipulate the sword freely. However, if you have not practiced long enough to learn the ways of doing so, it might be easy for your opponent to strike your sword down [and disarm you]."
This reinforces why this grip should only be used after significant practice. My own teacher would say "The sword should be wielded lightly, with two fingers." In response to question for more detail, the reply was always "research" i.e. practice, with the idea it would become evident with enough diligent work. (Some of us even speculated this cpould mean "ideally the thumb and any remaining finger" in the spirit of Musashi;) Luckily, this is explicated in Major Methods, such that one has a guide:
"The way to hold [the sword] is that you use your thumb, middle finger, and ring finger to hold the sword while keeping your index finger and little finger relaxed and separate, making the shape of the palm like a container that could hold something within it."
The note on the shape of the palm reflects the inclusion of Bagua in the Wudang system, where an empty, concave palm is standard across styles. It's important not to be too rigid in adhering to that until it becomes natural, because the had does need to be relaxed. The book makes the point: "It takes quite a long time to train to the extent of mastering [live-grip], and this can be taken to mean decades as opposed to years.
(Tai Chi practitioners will also notice a similarity of dead-grip and live-grip to how one makes a fist in external styles and internal styles respectively—in internal styles the fist is not tight but empty.)
Relaxation is key b/c "when practicing, Jing (focusing) originates from the Dantian, issued by Yao Ji (waist and spine), and reaches the [end] of the sword through the arm." If not relaxed, the power gets blocked by ones own body, but the book makes the point that "This type of Gong Fu requires both internal and external training" b/c one needs a baseline of strength to wield the sword in a relaxed manner.
Wrist flexibility is essential and also takes years to attain. Typically beginning student will cheat the grip to get extra range on certain downward cuts, but this leaves the wielder open to disarmament. One might see practitioners with the index finger hooked over the guard, but this is not recommended for obvious reasons.
My own teacher also added that the grip should be about 1 inch below the guard, although this may have been in service of the the draw from and return of the sword to the left hand in the standard "attention" grip (backhanded, blade controlled by the elbow) that many modern practitioners use at the opening and closing of the form.