I'm not going to fall on my head, of course. And even if I did fall on my head - it's not that bad. I've done it before and my skull isn't any worse for the wear. I am physically capable of dropping over backwards and putting my hands on the ground.
But I almost never do.
I wait til a teacher will come, stand, and pretend to anchor me so my head can justify diving over backwards.
I never realized before how much our heads could get in the way of things we're perfectly capable of doing. Think before you act isn't always the best policy. Periodically, my teacher will call across the room, "You! Do [crazy thing that I've never tried before] there!"
There are two reactions to this: one, which is what usually happens, is that I stop, try and process what he's saying intellectually, and wind up going, "..Huh?" and completely miss the point. Two: I don't try and understand what he's saying, and I trust he can see what my body is capable of and just do it. Almost always, with this second approach, I end up doing something I never expected myself to be capable of. Like Nike says - just do it.
My yoga teacher is a pretty wise guy. He asks good questions. And the one I keep getting lately, especially when I pop back up from a more-than-adequate hang back without touching the floor, is:
"Why fear you?"
(He talks like Yoda when dispensing wisdom.)
It applies to more than yoga.