I'm not exactly sure...I have been wondering that also. But if you go the the articles section it shows how to make a pillow cake. And they basically place the cake on an elevated surface and tuck the fondant under. I think that is what it says to do 
Check out the article, I think it will answer your questions. I hope other more experienced CC'ers will give us good info 
This is a brilliant question.I always wanted to find that out too!
I'd love to see the feed back this question gets.Maybe some experience cake decorator can explain it for us 
I dont know if this is the right way to do it...but maybe BC the part where it is going to be tucked in. It would make sense for you to BC the top and bottom. Then again....I might be wrong
Oooops...I meant to say "it wouldn't" make sense for you to BC the top and bottom of the cake
It's a good question, I haven't made one yet but I plan on doing one soon. I think you would butter cream the bit where you want the fondant to go underneath but not the "base" that sits on the board. that way when you cut it you are still getting the buttercream and fondant together. At least that is the way I think I am going to try but I may be completely wrong and someone who knows will set us straight lol. Not to hijack the thread but another question about the pillow cakes, you know the button dimples do you just press into the icing with a ball tool or something and then put the button in?
HanX
Probably depends on the shape of your pillow. I did a "squarish" sort of pillow, and I just covered it like a regular cake and after I cut it, just sort of tucked the very bottom edge under a bit. Mine was easier, though - because I did the "piping" around the edges, so if it wasn't perfect, you wouldn't see it anyway.
Hope this helps.
Oh - I used my kitchen ruler to make the marks, and used a small ball tool to make indent where I stuck the dragees.
I did one side at a time. Carve it, frost then fondant. Then carefully turn it over and do the same again. I used a rope to cover the seam. With smaller cakes I would do the same but when fondanting the top I wouldn't cut the extra and then flip, buttercream and fold the extra fondant over. That way they wouldn't be seams. Why no try it out and see what works best for you? Trial and error is a fun way to learn new things! 
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