A relative event is an event that is based on another event. For example, if you want your Twinkle to start flashing 5 minutes before bed time, you could use a relative event. The advantage is that if you later adjust bed time, the light still starts flashing 5 minutes before the new bed time.
You add a relative event just like any other, except in the When dropdown, you select the amount of time before or after another event you want this event to occur. Then you select the event to base it on in the next field.
Relative events can only be one level deep. You cannot base a relative event on a relative event. You can however, base any number of relative events on a non-relative event.