I think the idea behind FILD is that you barely move your fingers at all. You want to move them so lightly, you are almost just imagining it. Many people do the same thing with their whole body, imagining the feeling of rocking back and forth or falling, without actually moving. It helps to separate your awareness of your body from your actual body. You can try it now. Imagine lifting your arm, without actually moving it. It is like you send the message to move, without really activating any muscles.

This is really just an anchor to keep you from falling asleep all the way. Something to focus on as your thoughts drift into weird dreamlets on the edge of sleep.