The brushstroke is applied repeatedly, as the painter would have applied it to a canvas, nearly horizontally, sometimes left to right, sometimes right to left and overlapping strokes by about 25% horizontally and 50% vertically.
The reason I think of this as a systematic Rothko process is that the image is built up in thin transparent layers until the colour that is required has been achieved.
In the above image the background is built from three thin layers of pure red and the darker patches from three thin layers of dark red.
Go to SystemsArt.co.uk/RedOnRed to see a slideshow of the nine layers being applied.