Have seen a number of demonstrations in which the stylist puts the index finger of the left hand on the end of the comb when parting.
Looked and it appears the finger is behind the end of the comb and it seems to follow the comb in the part line. At the end of the part it pulls the hair away from the part line in one direction and the comb pulls the end away in the opposite direction. This seems to give a clean part in one stroke.
I have seen this so often by knowledgeable people that I am convinced this has to be an efficient way to form part lines with the fewest number of strokes.