It's fairly simple to set up a deadman det. The simplest way is to build the physical part so that letting go connects the contacts, rather than pushing the button. If the wires are cut on one of these, the bomb is safe because the circuit that fires the detonator won't be completed. However, it's not much more complicated to build a system that triggers when a circuit is broken, rather than when it's completed (simplest way is with a bypass switch, possibly using a voltage divider). Basically, the circuit is actually two circuits linked in some way (you can do it with some kind of processor, but a voltage divider is way easier). When one circuit (the one to the switch) turns off, the other (the one to the detonator) turns on. With this one, cutting the wires to the detonator handset would detonate the bomb in exactly the same way as releasing the button would. If you want to have it arm when pressed and detonate when released, then the voltage divider alon wouldn't do it - you'd need an extra (flip-flop, maybe) to stop it from firing the moment you hooked up the battery if you'd left the button un-pressed. Or you could just have a seperate arming switch, so you hold the button before you arm it. If you were using the slightly more complex processor-based method then it'd be easy to add the push-to-arm; it'd be a simple debounce.
