From the Youtube comments (you can take that with a grain of salt, though most articles give the same explanation)|
The reason why the sauce causes the tentacles to move after it is dead is because the nerves in the muscles don't stop firing until long after brain death. Sodium molecules react with the muscle fibers causing them to contract (like when they use sodium in the ER to try to jumpstart a heart which is a muscle) The muscles contract causing the tentacles to kick. You can also see this done with fresh froglegs being salted.
3DS: 0361-6951-2609 (Tom)