Do Not Hit Low
I have been studying and practicing Wing Chun for over 40 years. During this time I have come across so many different hands. At the beginning of learning Chi Sau 黐手, I did Chi Sau like I was fighting, later I did Chi Sau like I was learning more about defence than attacking, much later I did Chi Sau knowing how to use less energy and use my opponent energy so that when I attack the Chi Sau is over. It takes a long time to reach that level, you need to touch over hundreds or even a thousand hands. Teaching Wing Chun has given me a very good opportunity to touch many kinds of hands, strong hands, gentle hands, fast hands, slow hands, careful and wild hands. During this time I have come across quite a few Chi Sau techniques that I do not suggest people use as these techniques will damage your Chi Sau and distract you from improving your level. If you keep doing these bad techniques you will never reach a high level of Wing Chun.
The first bad technique, that some people like to do, is hit low. When four hands are rolling in contact, generally they will be chest height. Therefore if we suddenly hit low to the stomach area it means we give our opponent the opportunity to hit our face. Anytime we hit low, and remember our opponent might even let us do that, our opponents will immediately hit our face. So we should not hit low if we have not covered and are in control of our opponents other hands.