While working on "Birch Ballet," (see earlier posts), I realized that I wanted the distant tree line and grasses that butted up against that tree line, to blend and (in the case of the tree tips) "vanish" into the horizon. My tool was the tortillion, a tightly-wrapped paper stick.
Notice above that the tortillion helps spread the pastel into the abyss of the sky. It allows me to blend in the blues to the right and helps create a whispy look to the edges of the distant trees. Applying the tortillion upright (vertically) coincided with the growth direction of the tree line.
In the photo below, I am using the tortillion to merge the grass edges into the base of the tree line. This will also help to soften its edges as we know that light will emanate from behind the grasses and give them that faded look.
The tortillion also comes in handy when you need to soften a color and blend it into the background.
Tortillions can be wiped partially clean with a cloth but will buildup with pastel over time (and you can't sharpen them like a pencil, unless you desire to maim them to create some unique effects.