Secret? I read a long article about this "secret" program in Popular Mechanics about a year ago, and it went into fairly elaborate detail.
I did too. Basically it's the EMP (Electromagnetic Pulse) blast from a nuclear weapon, without the nuke. Pretty clever. It fries any electronics within a large radius that aren't protected, and it's hard to properly protect them.
To make one, in essence, you make a gigantic induction coil, similar in concept to the one in your car's ignition. When you're ready to fire, you charge it up to some ungodly electrical potential, then set off an explosives charge that vaporizes the coil, "setting free" its magnetic field. The main trick is vaporizing the coil at just the right speed, so that it destroys itself as fast as its magnetic wave is collapsing. This gives the magnetic equivalent of a sonic boom, if you will.