For those of you who don't want to watch the video:
Basically, when you put a DSi on to a DS Lite in a certain position, the DSi will go to sleep mode, regardless of whether it's open or closed. I've tried this with my DSi/Lite and my Brother's, and I get the same result every time. It also works putting a Lite on to another Lite. I can't try with a Phat/XL, so you'll have to find out yourselves for that!