The Smudge tool uses the current brush to smudge colors on the active layer or a selection. It takes color in passing and uses it to mix it to the next colors it meets, on a distance you can set.
You can find the Smudge tool in various ways :
through
→ → in the image menu,by clicking on the tool icon: in Toolbox,
or by pressing the S key on keyboard.
The Shift key places the smudge tool into straight line mode. Holding Shift while clicking the mouse will smudge in a straight line. Consecutive clicks will continue smudging in straight lines that originate from the end of the last line.
Using Ctrl with Shift, you can constrain the angle between two successive lines to vary by steps of 15°.
Normally, tool options are displayed in a window attached under the Toolbox as soon as you activate a tool. If they are not, you can access them from the image menu bar through
→ → which opens the option window of the selected tool.