Wacom Flex Nibs let you draw with a flexible pen nib made of rubber. When you put a lot of pressure on the drawing surface, you'll draw a thicker line than when you only press lightly onto the surface. That makes it easy to draw lines that vary in thickness. There's a lot of friction between the stylus and the drawing surface of your drawing tablet. Still, you won't feel as if you're scratching the drawing tablet, or that the pen nib will wear out because of the rubber.
Note: These pen nibs are suitable for the following styli: Grip Pen, Classic Pen & Airbrush.