Chattering is a way of easily adding a repeating texture onto pots that have been made on the wheel.  

The pot is positioned upside down on the wheel head and secured in place using ‘lugs’ of clay.  These are just strips of clay that are positioned around the pot to keep it secure and in place.   

The speed of the wheel is then set, and a chattering tool is positioned gently against the surface of the pot.  As the clay rotates beneath the lightly positioned tool, the chattering tool bounces up and down on the surface of the ware.   

When the tool hits the clay, it gouges out a small chunk of the clay and then bounces up again.  The clay rotates and the tool falls again onto the pot as it goes around.  Each time the tool hits the pot it gouges out a small amount of clay.  Because the tool hits the clay in a rhythmic way, the tool creates a regular pattern on the surface of the pot.   

Using a chattering tool is not essential and you can adapt a regular trimming tool to create a chattering effect.  

The difference between trimming and chattering is that when you trim your pots, you hold the tool firmly and evenly on the surface of the pot.  This removes the excess clay in an even strip and creates a regular surface. 

By contrast, when chattering the tool is held loosely and allowed to bounce as the pot turns.   

Altering the angle of the tool and the speed of the wheel will change the type of pattern that you create.   

Reference -  thepotterywheel.com -