I think there's more to it than that. My DD was resisting and barrel path was still messed up, triangle was not maintained. elbows got glued to her side. Maybe you are just doing it naturally or somewhere along the way you learned to maintain the triangle.. I believe it's more than just resistance.
This is interesting. I say: do both -- engage your hands AND your stretch (you emphasize 'core' I'd describe stretch creation a little differently) rather than either/or. But your feels are your feelsIf the hands and upper body resist as long as possible, wouldn’t the triangle be created and maintained? I swing too and when I resist hard with the upper body my obliques engage like crazy and my lead elbow works higher. Once I start thinking about thinking about a hand movement my obliques disengage and all my focus goes to my hands. That’s just my swing feelings