So now you’ve seen the entire process of creating the fore-edge painting; here is the video of the ‘surprise’ revealed on fanning the edges. I’m less good at fanning the edges, better at painting them!
The grip that’s used to draw straight lines against a ruler. Fifth finger for balance and you don’t have to ‘paint’ the line, you just pull your hand along the brush (using four fingers as shown) and it draws it automatically for you. One day, with an extra pair of hands, will try to do a video. Needs practice, and ruling pen is easier. However – you won’t get a finer line than that drawn with the hairs of a brush.