Unfortunately I have no grounding in proof so whatever I say won't matter much compared to overlays etc, but I used to trace exactly in this way when I was 15 or so; I would Frankenstein or trace "just the thigh" or "just the chest/armpit area" etc. Tracing "just the torso" is absolutely something that's done by people who aren't quite as confident. For me it's always been shoulder areas, though I would never trace nowadays, I'd trace shoulder/neck/chest regions back 9-10 years ago without hesitation. Additionally it's blatant that they are "inspired by" the poses that the match-ups come from.

Either way it doesn't matter in the long run, as the majority have deemed it not tracing. But I have absolutely seen (and taken part in) this sort of tracing over the years.