Use a fude style brush (Japanese calligraphy brush). Load it fully with pigment. Tag it down onto the surface and press lightly...... then as you draw the brush over the surface lift up slightly to reduce the pressure (and the thickness of line). As you near the other end of the line, start putting slight downward pressure on the brush. Stop moving the brush while there is still downward pressure on it. Lift straight up.
Practice this with red iton oxide and water on a copy of the NY Times. By the time you have filled a few dozen pages with lines... you'll understand how to do it.
Learn to paint the classic bamboo stalk used in sumi painting.... and you'll understand how this effect is done.