I tried this recently by blowing air into me - it was quite hard to do! - and could feel the effects immediately. After quite a few cheekfuls of air had gone into me, I felt quite uncomfortable, very "stretched" and bloated. I didn't really find it very comfortable, and it took ages to get the air out. I don't know how these people who blow themselves up huge with compressors can do it, it must be really painful surely?