Ashley hit it already. Pressure points. Also, just the positioning of the bones or ligiments in the right way/angle creates the pain. You want to get on your tippy toes to try and avoid the realigning of the bones or ligiments to take away the pain.
Here's a vid of Okamoto of Roppokai Daito ryu:
Starting at 0:17 and specifically at 0:27-0:30, could you tell me where the pressure points are that Okamoto is using?
While I've never trained with Okamoto, I highly doubt that he is simply using pressure points to affect his students. I'd believe more along the lines that Okamoto was using Daito ryu aiki.
As for demos ... well, Japan is famous for them ... in more ways than one.