The hyun sau movement, or circling hand movement, is a movement that comes quite often in the Wing Chun forms. A lot of people are a little confused about this movement because it comes so often. Some people worry solely about the practical explanation and practical reason behind it. When you see the hyun sau done in the form, it's done something like this: after the punch, you open the hand into tan sau, or sometimes you go directly into it, and you fully circle your wrist, and then you pull it in.
There are a number of different variations on how to do this, depending on what your lineage of Wing Chun is, how your Sifu does it, and even within one family or one lineage of Wing Chun, there are often variations. What's very important is to understand the main idea behind it. Of course, a lot of people tell you, "Well, if somebody grabs your wrist, one of the main functions of the huen sau is to get out of a wrist grab." Certainly, that's a practical application, of which there are many others as well, but I don't think the founders of forms in Wing Chun put a circling hand at the end of every set because they were hyper paranoid about having their wrists grabbed. This is only one potential thing you can do with it.
In fact, one of the main functions behind the hyun sau movement is, because when we're fighting on the street, we don't have our wrists wrapped up. It's not like boxing; if you go to punch somebody, you wrap up your wrists and then put gloves on and everything is nice and protected. When you fight on the street, you have to fight with your bare fists. You have to fight with your wrists completely unsupported, so what we need to do is we need to create a backup support system to make sure that the wrists are strong enough and flexible enough to hit at various angles, because you're never going to get a clean, flat shot like you would if you were all wrapped up or you were punching a board. You're going to have to end up punching at off angles and things like that, so one of the reasons we do the hyun sau is to prepare your wrist for this.
When you practice a hyun sau, it's very important not to cheat. You see, some people when they do hyun sau, they just open the hand and close it, and this is a fairly useless endeavor. When you practice the hyun sau, you want to keep your arm absolutely locked. You're going to bring the wrist all the way back without letting this bend. A lot of people will bend here, you want to keep it locked. This goes all the way down, and in the bottom position, one of the secrets, especially for training your tendons and training the muscles in here, is to close the fist in the bottom position. Many people close later at top, but if you really want to get the full benefit, come all the way here, keep your shoulder down, elbow locked, close the fist, and then bring it up. That's the standard version. It's also a good idea once you bring it up, if you want to turn it in this way here, so you can get a little bit more wrist flexion. That's why I teach my students to come down, close the fist, come up, and turn it in, so we have a full range of motion.
Some Sifus just teach the students to close the fist and bring it up, which is still better than closing the fist here. Ideally, you want to close the fist and bring it in. One other point, too, if you can keep your elbow in the same place, you're going to get much more of a stretch on your forearm than if you let your elbow bow out, which is very common. You see a lot of people circle and they let the elbow fall out like this. Ideal is you keep the elbow here. That way, you can control your wrist independent of your elbow, which is a very important thing to do in Wing Chun. We want to be able to articulate and control our joints independently.