I thought it was a good film for what it was. This kind of violence isn't something I like, but the film was very creatively directed and executed and there were some very imaginative visuals. The director, I gather, was trying to make ultraviolence beautiful, and while I don't think that's entirely possible or desirable, I do respect the technical and artistic ingenuity that went into it.
I was surprised by how ethical Dredd was in his way. He killed without remorse when he believed it was necessary, but refused to kill a suspect unless he was 100 percent certain of guilt. True, the fundamental flaw in his reasoning is that you can never been 100 percent sure of guilt or innocence, and that being both arresting officer and judge and jury is a conflict of interest that makes an objective judgment impossible, so the whole system is fundamentally unjust. But while Dredd makes the mistake of trusting the system, he's nonetheless scrupulously ethical in his obedience to its rules, and even shows some moments of willingness to bend the rules ever so slightly for the greater good, like when he lets the kids go.
^This is one of the main things that showed that the filmmakers "got" who and what Dredd is about and exemplifies how faithful they were to the comics.
One of Dredd's defining characteristics is that he's a true believer in justice but at the same time, he's not blind to the weaknesses of the system. Quite the contrary, in the comics the only reason he's still a judge at all (having already taken the Long Walk) is because he doesn't trust the system not to deteriorate into a chaotic oblivion (which says something, giving the nominal status of Mega City One is about five and a half steps beyond pandemonium!)
IIRC he (privately) refers to the whole thing as the "big lie."
I admit, it's a handwave but...
the only way I see it working like this is if it's a type of deadman switch that only transmits a detonate signal when the sensor fails to detect a pulse. So dropping it out of range would essentially neutralise it. I'd like to think this clearly flawed design was cooked up but that techie/mutant kid as a way of sticking it to her, but it must be a known thing since Dredd clocked it's function at a glance. Maybe it's meant to function as a medical device? Something they pilfered from the bloc infirmary, like say a remote monitor to track when terminal cases and oldsters have expired in their apartments so they can send in the droids for clean-up and transport to resyk?
Honestly though, it's probably just something we're not meant to think too much about.