Electromagnetic boundary conditions across a totally reflecting surface lead to a wave that dies off exponentially (within a few wavelengths) beyond the surface, but does not carry energy across it. However, this wave can be transmitted if it is made to interact with dense matter sufficiently close to the surface. The demonstration shows the hypotenuse face of a right angle prism acting like a perfect mirror, except for a spot around the point of contact with a second prism. If the angle of incidence (on the hypotenuse) is close to the critical angle (42 degrees), a bright spot will appear where the two prisms close to within a few wavelengths, allowing the wave to tunnel through.
The spot can easily be seen on the overhead projector.