Enemies have animations for blocking and dying to attacks from behind, so it works when you block and spin past them, or otherwise get yourself behind them. If your sword is behind an enemy and you throw it at their back, they instant-spin to block it as an attack from in front, though, and a lethal hit spins them just before the sword hits too.
That said, very cool new feature, and got more chances to test bugfixes from previous updates and all seems good. Also, really liking all the new sounds. The one thing about the audio which feels least good to me is that the kick sounds the same even when you're following up the body split attack. That one seems like it probably deserves a different sound.
Also, when you teleport to the sword after a bounce, teleporting into the air locks you into your grounded stance, and you can't attack until you land. Also, if your sword is on the ground and you try jumping over it, you get pulled down onto the sword instead of being able to continue over it. There's no way to get past your sword without retrieving it. Feels fine in other interactions, but weird on the jump.