Nian: Grinding
In taiji we say "Sink down to grind out". This saying refers to the action of this device. It is called a Nian.
Sink down, is when the grinding stone goes over something, it does not float up lightly, instead, it will go over within downward sinking power. It moves forward leaving a trail of powder behind!
In addition, when we push this grinding stone forward parallel to the the ground, our forward power is converted to an vertical power through this device called Nian. This is taiji conversion!
Today we have grinders
Hong called it "gear box"
Old days they were called Xia Ta Wai Nian!
Source: https://practicalmethod.com/?p=64378