It is told by almost any teacher that the wrist strength is important for making the strikes sharp, and hence validating a point. However, its significance is much further beyond the mere sound effect. The following is a list of the other benefits:
- It keeps the upper body up-straight. A lot of young kenshis (for example high school students) and beginners whose sole interest is winning tend to let the upper body lean forwards when striking men, in order to add an extra speed and strength to the strike. If this can be achieved by the wrist movement along, the leaning of the body can be avoid. This of course looks better, and is also essential if one were to make continuous strikes.
- Better ki-ken-tai-ichi and more explosiveness. These are direct consequence of the previous point. Because if one keeps the body up-straight during seme or at the initial stage of the strike, it is very difficult for the opponent to detect when the strike will come.
- Shikake-wazas rely heavily on wrist strength, as elucidated by Ozawa Sensei when I was in Tokyo. They include harai, maki, osae, and of course kote-men, etc.
How to increase wrist strength?
Many ways. In fact, I have seen many people telling each other special exercises on how to increase wrist power, but I think the most effective exercises are the OLD SCHOOL STUFFS: kirikaeshi, renzoku-waza and continuous men-strikes raising the shinai to the "medium" position (see here for explanation). While doing them, make sure the arms and the wrists are fully extended. Especially the right index finger, if straightened, should point downwards as much as possible, instead of pointing horizontally (with the right arm being horizontal).