"Microwave Susceptor". Its a super thin layer of aluminum or graphite that absorbs microwaves and gets really hot. Its in the bottom layer of the bag and the reason for "this side up".

Interesting how I can get almost every single kernel to pop without burning in a plain paper bag.

Wow, I assumed similarly - that there's something in the bag that helps the air inside heat up. I guess maybe moisture in the corns is enough. I learned something new and useful today, thanks!