The Science
The application of a stun (110 Volts - 2-5 amps) causes an immediate interruption in the functioning of the nervous system of the shellfish. By interrupting the nerve function, the shellfish (be it Crab. Lobster or other) is unable to receive stimuli and thus by definition, cannot feel pain or suffer distress (Dr. Dave Robb 2000 - Bristol University - paper on sentience in Crustacea, Baker 1975, Jane Smith 1991, Bateson 2000, Sherwin 2000 & Gregory & Lumsden 2000). The prolonged application of the stun causes a permanent disruption which kills the shellfish.
Legislation
New Zealand & a number of Australian Provincial States include Crabs, Lobsters and Crayfish within the definition of animals requiring humane treatment in handling and killing; NZ; Animal Welfare Act 1999.
It is unclear whether Crabs, Lobsters and Crayfish are included within the current UK Animal Slaughter Regulations. They are however likely to be specifically included in the Animal Welfare Bills currently going through the UK & Scottish Parliaments. This would require them to be 'humanely killed'.
Similar legislation is being contemplated throughout Europe.
Read about Crustastun on the web:
Specifically developed for restaurants.
The Continuous Flow for Processors.
More about the Continuous Flow Stunner