Actually, they are the same. During the early days of my studies of Wing Chun, I did not focus on all of the names of the Wing Chun hand techniques, which is like everyone, we focus on the skill and learning how to do things. So whatever my Sīfú called a hand technique is what I called it. At the end of Síu Lihm – 小念頭 – Reducing Thought, before the Chūng Kyùhn 冲拳 – Punch, we hold one hand down and the other hand, palm up near the elbow. We then wipe that hand straight down and then swop the hands round. This is Mut Sáu. However, when I started to read other Wing Chun books they used the name Cháan Sáu, and even a book written by my Sīfú, the golden covered Wooden Dummy Book with my Sigong, Ip Man, this also used Cháan Sáu. So I began to use the name Cháan Sáu.
Mut 沫 means wiping, like wiping a table. One hand wipes down the straight arm and this is the reason it is called Mut Sáu. Cháan means shovel. So the hand is shovelling down, so we also call it Cháan Sáu. Therefore we can use both names, but I prefer Mut Sáu