Complex sentence: Due to the shape of the pointed nib thick lines can only be produced on down strokes; if pressure is added to the pen on an upstroke the nib tines are likely to dig in to the paper.
Split into simpler sentences: Due to the shape of the pointed nib, thick lines can only be produced on downstrokes. If too much pressure is applied to the pen on an upstroke, the nib tines are likely to dig into the paper.