When you are plummeting down a steep drop, the various bits of your body are weightless, each falling individually inside your body.
This is what gives you that funny sinking feeling in your tummy, like the sensation you get when you're in a lift going down quite fast.
What you see with your eyes (if you've not got them clamped shut!) adds to the excitement. Going upside down, things whizzing by, the air blowing through your hair - the perfect ingredients for a pretty cool ride!