The implosion had to be very symmetric or the plutonium would simply squirt out at the low-pressure points. This means the implosion had to start at essentially the same time over a large part of the surface of the explosive material. In other words, the detonators had to very precise, timewise. Alvarez and Johnston achieved this precision the only way possible: simplicity.
An EBW has two main parts; a piece of fine wire which contacts the explosive, and a "strong" source of high-voltage electricity—strong, in that it holds up under sudden heavy load. When the wire is connected across this voltage, the resulting high current melts and then vapourises the wire. The resulting shock and heat initiate the high explosive.
This accounts for the heavy leads seen in photos of the Trinity "Gadget" (does WP have one?); they had to deliver a large current with little voltage drop, lest the EBW not do the phase transition fast enough. (This potent electrical source must've been something of a challenge in a deliverable bomb, however. Anyone know about it?)
EBWs have found uses outside nuclear weapons, such as the Titan IV.[1]
