Basically it's a way to sleep in without sleeping late, by adjusting when you go to bed so that your body wakes up naturally just before the alarm.
I write down when I get in the bed. I play guitar, read, whatever, just no tv or anything that keeps me awake.
I write down when I get out of bed. NOT WHEN I WAKE UP, WHEN I'M OUT OF BED. My alarm is 5:30am. If I beat the clock by 10 minutes, tonight I'll get in bed 5 minutes later. Waking up and immediately getting out of bed without the alarm at 5:29 = win.
It's the waking up to the alarm that's bad for you. Now my alarm is my "get in bed earlier" alert.