If it is of the "flip-disk" type the applied voltage is only needed briefly. Basically the disks are metal plates either permanently magnetized or have a small magnet attached to them. When the solenoid is powered the disk will flip to align with the solenoid. They are operated as a matrix with parallel address lines and vertical columns.
These were quite common for outdoor tote-boards and similar, used in train stations, airports, etc.
They have seen a bit of a revival, using retroreflective disks and led's in the corner to illuminate the disc. 3M makes them for traffic control signs. At my work we have sold some as finished units, never really got to look inside. The appeal is that they only need power to setup the sign, and low power for the illuminating led's, compared to making the whole display from led's.