I've posted a new video at http://www.youtube.com/watch?v=iDN1Tbsxkec that shows one sort of illusion you can make on a flexagon.  I start out by showing how you can imprison the king by performing a simple flex.  Or, if you happen to live in a country that hasn't made its reputation by overthrowing the monarchy, the end of the video shows how you can free the king.  It also demonstrates how to fold it.

You can find the pattern and folding instructions at http://loki3.com/flex/maze/silver-tetra-illusion.html.

I put this illusion on the "silver triangle tetraflexagon", a flexagon with four 45-45-90 triangles per side.  This is where I got the name for the "silver tetra flex", being the only flex you can perform on this flexagon.  The illusion works nicely on this flexagon because the faces with the king and the walls are always visible, so you know it's not using hidden faces to accomplish the transformation.

Scott Sherman