The open cell foam inside is allowed to expand which literally sucks the air in - you can also top it up by blowing some more in if you like things a bit 'firmer'. Made from 70D nylon its a robust little rascal but if you do try sleeping on a bed of nails a repair kit is included as is a snazzy stuff sack for storing it in. Packs down to a fairly respectable 75 x 22cm in size.