Sorceror said:
I seem to recall watching that sequence going "that's clever, the way they shot it, it looks like 3PO is heavy enough to make those grooves".
As I recall, Anthony Daniels is such an incredibly good actor (possibly the best the world has ever known, I'm sure he'd say!) that he allowed himself to be crushed and twisted inside the suit for those scenes. He then used some sort of self-taught massage technique to make himself normal again.