Switchable Metamaterial Makes Itself Invisible - Technology Review: The metamaterial in question consists of a periodic array of split ring resonators, c-shaped pieces of copper, the size of pennies. Any field of the right frequency induces currents in these split rings that resonate back and forth. This resonance, in turn, interacts with the local field focusing or bending it.
Lopez and co have modified these c-shaped pieces of copper in a clever way: they've added a pair of diodes that close the 'c' making o-shaped conductors. In a strong field, the induced currents in the rings pass through the diodes, effectively shorting the split rings. When that happens, there is no resonance and so little or no interaction.
In that case, the properties of the metamaterial are similar to air and the metamaterial is essentially invisible to the field.