Thanks for the further precision. You confirm things that I suspected by pouring over the grammatical structures. "Pie1 ji1" may cover slightly different territory than "pie shen," or else the name would have been "zhuan shen pie chui." (or could it be an issue of rhythm and cadence?)
It is amazing how all languages have difficulty expressing precision about movement and instead rely on context and daily experience for clarification.
By the way, I find the English for this posture to be highly misleading, imprecise, or even inaccurate. For me, a "chop" cannot describe a "back fist," but rather strongly implies an open hand slice, as if with a butcher's cleaver (e.g., a Karate chop). I also would not describe the movement as a "punch," since, for me, a "punch" describes something that thrusts and often attempts to go through, not something that smashes or pounds. For me, punches can be described by pounding only when they are repeated in the manner of a pestle pounding in a mortar.