A little while later someone posted an explanation for the phenomenon:
When a creature dies, its neurons don’t stop working right away. So long as there is still enough energy around to maintain that membrane potential, the neurons will work.
What you might have noticed is that in the case of the dancing dead, the cooks have added one key ingredient: salt (soy sauce is very salty). Salt – or sodium chloride – is chock full of sodium ions. This overwhelming dose of sodium ions is enough to trigger the still-working neurons into firing, signalling the muscles to contract.--Christie Wilcox
|A close up image of octopus arms showing suction cups. Image: seasky.org|
Death is a notoriously difficult term to define, and we're not entirely sure whether the squid in the video was already dead when poured with the soy sauce-cephalopods like octopus can live out of water for 30~60minutes. So for the sake of brevity, let us just assume that it was already dead by the time it's mantle was cut.
If you've got enough experience in cooking, you'd probably know that frozen octopus arms, even those that had been dead for more than a day, could still move-albeit just a little bit-when you pour hot water over it.
Hence, if the squid had in fact died, what we saw in the video was not an actual "pain" response from the it's brain, but rather a natural response from the neurons that didn't "die" fast enough, and thus responded to the pouring of the soy sauce.