>>29093282
Crab meat starts to become ammonia-toxic, starting from the carapace, immediately after death. You boil them alive, they die immediately and such a process is immediately halted as the meat flashcooks and also halts bacterial proliferation entirely.

Of course, the best way to kill them like that is to first put them in ice water for a few mins to stun, then fully immersed into a raging boil pot that can be fully submerged in, close. Crab won't even be aware it's gone.