Just about all fountain pens came equipped with what are now popularly called "flexible" (or "semi-flexible") nibs when I was growing up in the 1930s and '40s. Nobody knew they had a "flexible" nib in their pen and they didn't attempt to develop a technique of consciously producing pressure induced thick and thin strokes in their writing. Of course, everybody I was acquainted with used a light and relaxed pen hold touch when writing -- a technique that seems to have become generally lost -- the nuance of letter form line weight was incidental.
I think the general public just used their fountain pens as they had their school dip pens -- some wrote with light paper contact (as I always have) and some wrote with moderate pressure and produced writing with nicely "shaded" (as they say these days) letter forms. Others wrote with heavy pressure and produced noticeably (but often erratically) "shaded" writing -- and lots of blots. Heavy pressure writers also risked ruining their nibs -- all that excessive tine spreading eventually took its toll -- fountain pens saw an enormous amount of daily use in those days.
Modern users of fountain pens with these "flexible" nibs can write normally with them -- as they do with any fountain pen -- but with a relaxed and light hold -- or they can use them to produce deliberately "shaded" writing by developing a conscious "pressure on the nib" technique (or even employ them for Copperplate writing).
I derive my own greatest pleasure in using these nibs from the soft feeling and tactile feedback that I experience when writing on fine paper (the subtle line variation is incidental). I believe that is what they were originally designed for.
The following exemplars were rendered using vintage flexible nib fountain pens and the light pen hold and paper contact I outlined above.
James