Another Octopus customer who's done this a few times now.
It's not necessarily less electricity, just less in the specified period to spread out demand at peak times. They want you to start cooking dinner at 6 pm instead of 5 pm for example.
It's not a huge amount of money. This afternoon will be our 7th session and so far, we've earned £4.22 (plus whatever we get for last Thursday morning that's not been credited to the account yet). That's by cutting down between 20 and 80% in each session.