It's a neat and inventive mechanic, which seems like it would have room for plenty of interesting interactions and mastery in a full game. What's there so far is a fun prototype, but a little on the easy side. I mostly just used the knives to hurry through the boring parts at the end of each stage/wave where there's only one enemy and you're just waiting for it to fire. Having another way to speed through those last forgone-conclusion enemies would be good - there's no interesting decisions or challenging gameplay in those parts, because a single enemy (or two) is something you've already proved you can beat after the tutorial.