Basically, the way that these pillows work is that when they are heated, they expand (the plastic will melt away) and fill the void. This maintains the fire rating and prevents fire from spreading and smoke spread is contained.
This is an old fashioned foam eductor and a bucket of foam. When is the last time you had one of these in your hands? How about the last time you actually flowed one? Well, unless you just completed a foam class or a Firefighter 1 & 2 class, it has probably been a while.