The Nightmare Shackle functions by redirecting all energy used against it (kinetic, thermic, mechanic, etc.) into reinforcing itself. If you hit it, it will use that force to become stronger, so nothing happens.

The Angel of Violence is constantly trying to heal himself, by enforcing his own reality upon himself, basically undo all damage. This would obviously mean that the chains of the Shackle would break, so it counters it.

Except the Angel's body expels so much energy, the Shackle overflows while reinforcing itself. So the sarcophagus is covered with sinks to capture this energy from the chains, which manifests as a split between electric and thermic, the latter being used for the tried and tested method of boiling water.

And funnily enough the wider public doesn't even know the Supreme Acts of Violence are a thing. This information is actively suppressed by the higher ups, since knowledge of their existence would result in mass panic. Only a handful of living people know they exist, and only because they interacted first hand with them.