Silviya, that is called "Density Control", and is a switch for pressure sensitivity in your drawing program of choice. I have it usually off, unless i want to get some non-uniform gradients, shading and/or texture going. Alternatively it is also useful in emulating an actual pencil if you set it up correctly. (The brush size then decides how smooth things are. The bigger the smoother. Or you can say "fukkit" and blurr the color/shading layer(s).)



Both are done with 4 and 8px pens and Density selectively enabled.
There's benefits to both methods, relating to mutability and style. Kinda depends on what you're going for, really.