If you're already pubbing and not at home - you may not save much. They compare your usage during the specified hours to your average usage during those hours before. That is - tonight, they'll compare your usage to 4.30-6.00 over the last 10 weekday 4.30-6.00 periods.
Been doing it for a while now with Octopus, and earning 'Octo Points'. Haven't earned many - because we only use c3.5kWh in a day anyway. And we don't get that much back per kWh - £4 I think.