The idea behind this method is very simple, instead of clearing the drawing area, we are continuously drawing semi-transparent rectangles that cover old drawing states more and more. This very simple method can give you interesting effects with particles. You can also manipulate the opacity of each rectangle by changing the alpha channel of the rectangle color, which becomes a color of the background.

To draw a tail with lines, we have to store several particle positions and draw a line through these locations with variable opacity. The rule for opacity is just to draw older locations with less opacity. You can see the drawing code and alpha channel calculation in step 4