Basically this means that I can decap a gorefast at the furthest katana range by aiming at its headshot hitbox
It's a shame that you have to say "head hitbox" because said hitbox is not actually ON THE HEAD... Please fix, TWI...
Anyway, yes, the sword actually swings it counts as a "Hit" when it connects. So often, when doing that "Upwards" swing of the Katana, you'll hit an arm, torso, or leg if something is RIGHT next to you instead of a step or two away as the sword moves up. When it hits somewhere, that's where the hit is registered on the critter and the sword will pass through the rest of the way without doing damage. Usually still does enough damage to kill, though, but it's why you often see arms and legs getting chunked off of things when you clearly aimed for the head.
Irritating, though, because it'll often happen when you swing for a charging Fleshpound's head; you'll end up hitting his arm or body and still get hit instead of hitting the head for the decap.
I suppose it's probably an intended "Disadvantage" vs the chainsaw, as it's MUCH easier to decap with the chainsaw because it instanty hits straight where you aim with no 'Swing', save for the alt-fire. As many will agree, this makes it MUCH easier to decap things (Like Fleshpounds!), but it's still no match for the Katana's move speed and insane damage...