How on earth does this work?

[FrustratedFinFan]

Here is my take on it...

The vibrations come in waves, radiating out from the source. The metal (or other chosen surface for the board) acts as the medium through which the waves travel. In the case of metal, the material is a good medium for the wave propagation to take place. Add a flexible magnet swatch under an active spot, or duct tape, and that material serves to dampen or break up the wave pattern in that area....It might be a combination of the tape and/or magnet not being a good material for wave propagation....as well as the added weight inhibiting the motion/vibration of the metal....not positive there.