Basically you go too bed really late and get up really early so you retrain the brain. It is a lot more scientific than that and you also have to get up in night if you have been awake for more than about 15 mins.
You recondition the brain to associate the bed with sleep rather than wakefulness and tossing and turning. You force the sleep into a shortish window rather than the classic mistake we insomniacs make of going to bed earlier to try and catch up on sleep.
Its paradoxical - in the same way that low doe mirt makes you sleepier than higher dose mirt.
It is beneficial for about 2 thirds of people that do it, but obviously you can be really knackered in the earlydays of doing it.