No deep mystery the shape allows the strong clips to rest unstressed I have always opened a clip by hand when putting in my top pocket of the shirt. Some pens never see a pocket at all get scrinched moving the ball part of the clip against the pen top whilst winding in and out the biro marking the timber a fact I do not like at all.
My design feature far as I know have not seen this done by others I like the design it enables from boring end to end. I choose which pens to do this with.
Kind regards Peter.