"If your knees are bending then all your muscles aren't clenched. It's the quadriceps, I believe, or that's what some of those muscle-bound sword-swingers call it. Their's tend to be rather hefty."
Can you justify it? Sure, why not. Human beings have justified weirder stuff for fun, profit, and religion.
But why the nine hells should I have to? I've got better things to do than to write fluff for the designers. Fluff that they're fully capable of doing themselves if they took their brains out of the number-cloud for half a second and thought, "Wait a second,
does this make enough sense?"
Effect-based design like that is boffo for balance, since the designer has total control over the limited uses of the ability and they don't have to worry about anything unexpected or surprising happening at all.
Of course, IMO, it's regular bollocks for
fun, since loosing control and having unexpected and surprising things happen is part of what fun is. And it's bollocks for immersion, since it gets the chronology of physics entirely backwards. "Here's the result, you figure out how it happened" is
not how the world works, so it's not good for imagining how this imaginary world works. There's no cause and effect, it's just effect, effect, effect, and I suppose you can interrupt that chain of effects and hypothesize about the cause if it makes you happy, but since those causes have no effects themselves, it's basically pointless.