The History Brush 
partially or fully restores the processed image layer to the selected state, weakening or canceling the applied changes or effects.
The basic tool settings are shown in the Tool Options panel above the Image Window or by right-clicking on the image. To display the full list of parameters, press the tool icon in the Tool Options panel or use the F5 key. To specify the parameter settings, enter a numerical value in the parameter’s field or use the slider.
Element - in the drop-down list, select the type of the basic element of the brush: Ellipse or Select Shape.
Size (1-5000). The maximum width of a line made by the brush (in pixels).
Hardness (0-100). The degree of blurriness of the brush’s edges. As the value of this parameter is increased the brush’s edges become harder. At values near 100% the border between the brush’s edges and the background is very distinct; at lower values the transition between these areas is smoother.
Hardness = 40
Hardness = 90
Strength (1-100). The degree of restoration to the selected state. At lower values there will be less restoration and more blending with the effect; at values near 100% the image will be restored completely.
Strength = 25
Strength = 75
Fill (1-100). The parameter affects the intensity of the Strength parameter in a single brush stroke. At the value = 100%, the Strength parameter is not affected.
Fill = 25
Fill = 75
In the Restore to drop-down list, select one of the previous states of the image. Areas processed with the tool will return to this state. By default, the brush restores the image to its initial state.
In addition to the above-mentioned parameters, you can adjust the advanced settings.
Press the Default button to set all settings to their default values.
To draw a straight line, first specify the starting point with the left mouse button, then, while keeping Shift pressed, move the cursor to the desired end point of the line and release Shift. If Shift is not released, a new line will be drawn to each point where the mouse is clicked.