In chapter 838, Cracker put a full weight slash from the side onto an extended arm using his DF to trick Luffy's arm into such a prone position.
That is not effortless, that is a full power sneak attack slash that didn't breach bone or inhibit Luffy's offense.
It matters because without bizarre sneak attacks Luffy's arm has never been slashed prone like that, even against the worlds strongest swordsman... Now Luffy has FS he can't be sneak attacked without PIS which isn't relevant for power scaling.