It makes no sense to me either. Lying on one's back is "supine" not "prone". "Semiprone" usually refers to person lying on his/her side.
Student or Learner
So I read an article and stumbled upon this sentence: "Instead of sitting in a conventional seat, the driver lay on his back in a semi-prone position."
I'm confused. Isn't "prone position" meaning you lay flat with the chest down and back up? Then why the driver lay on his back in a prone position? I don't get it.
I am not a teacher.
I think this is a simple case of the writer not knowing the right word.