I can't figure out how to activate the spin blade attack (after collecting it in room 4905). I keep double jumping and/or dodging, but this doesn't seem to have any effect, I cannot harm enemies and my steam meter won't recharge. I seem to be stuck in 4703 now. What did I miss?

I figured out what happened: I made it through 3409 without the homunculus, so I got the spin blade at a time when I probably wasn't supposed to have it.

To get out of the dead end in the toxic swamp without the homunculus I needed to temporarily enable the co-op mode and create some blocks that allowed me to travel over the toxic goo. Once I found the homunculus, spin blade also started working.