I think it is an electromagnetic field (EMF) that gives the pieces their transparency... Once the pieces touch the board (with is "electrified") the EMF forces the opaque material to fiscally change into transparent.
When removed from the board the piece is no longer under the effect of the EMF and the material changes back into opaque. So you an say tht the substance has memory properties.
This tech is being developed for windows (the glass ones, not the OS, lol!). 