Short answer: we'll work out what you typically use during the trial time slot based on your historical smart meter readings, and calculate a % we'd like you to reduce that by for each event.
Long answer: We'll look at your historical smart meter data to work out the average you used over the same time slot for the past month (so if we want you to Turn Down at 4-6pm on a Monday, we'll work out your typical energy use based on what you used between 4-6pm during the four previous Mondays).
We may set your target reduction differently to other customers as different people have different abilities to shift, e.g. if your evening consumption is normally 2kWh, we might ask you to drop to 1kWh, but if you're a typically low user we might ask you to reduce by a smaller %. Any energy you use below this target will be credited to your account at the unit rate of your supply agreement.