Left-handed writers fall into two groups - those who write below the line, the same way that right-handers do but obviously with the left hand, and those who write above the line, with the paper at a jaunty angle to avoid smudging.
The tip of a left-handed nib is ground the opposite way to a right-handed nib.
The first group will almost all get an obvious benefit from a left-handed nib, since they are holding the pen at an opposite angle to a right-hander. The second group may or may not get a benefit, although most of them will, and that will depend on the angle at which the pen is held - some left-handed overwriters hold the pen at the same angle as a right-hander and push the pen, and some hold it at an opposite angle and pull the pen.
It is not the pushing or pulling that makes the difference, since a nib is supposed to be able to write smoothly in all directions, it is the angle at which the pen is held.
