You need to practice a lot, it took me 4-5 days before I started to get the hang of it, and now it's pretty effortless after a couple weeks.
My 25+ years of guitar playing may have helped, as my motor skills with my hands are pretty good.
I flip the razor with my thumb and index finger and make sure I have pretty much the same grip and pressure on the tang going both ways.

I made a quick little video to show how I do it, excuse the crappy webcam quality...
I'm doing it really slow at the end, so you can see how I work my fingers:

Hope it helps.