When you pick up an artist's paint brush for the first time, chances are you'll hold it in your hand in the same way you would a pencil or pen, with your fingers close to the ferrule. If you never hold a brush in any other way, you'll be limiting the range of marks you create with a brush.
Holding a brush like you would a pen for writing is the most instinctive way to hold a brush, as it gives us a familiar sense of control. Using your wrist to move a brush held this way provides fine control, suitable for making precise marks. For even greater control, some people rest the side of their hand or little finger on the canvas (protect your work by putting a sheet of paper under your hand if necessary), or use a mahl stick. For more flowing marks, use your whole arm not just your wrist.
Remember, you don't have to move a brush from left to right as you do a pen or pencil when you're writing. You can move it whichever direction you want -- and you'll paint better for doing so!
