Yup, so a 1 - 2kW electric fire could be switched by a local transmitter triggered by the supplier.
If not voluntary, I know many will go for it for the benefit of the masses and others won't because no one is forcing them to and they are ok (Jack).
What would be nice is smart meters that if they don't detect some level of reduction at the required times (assuming a usage above a certain threshold etc) they cut *all* the power off. ;-)
Those affected won't care as they will already be out ... in the pub and parked in a disabled or parent and child bay.
Cheers, T i m