Scheduling high power appliances inline with a smart tariff (e.g. Octopus Agile)

[Brian The Snail]

I've read many posts where people with smart electricity tariffs say they schedule washing machines, dishwashers etc to come on when they consider the cost per kW to be in their favour.

I can understand how this works if you turn on the machine and schedule the machine to run at a certain time, but how does that work if supply to the machine is only switched on when the trigger cost/kW is reached?