Quite a few Osprey's have mesh ventilated backs. They call it Airspeed and Airspeed Suspension Back Panel. I'm guessing that the Airspeed Suspension is the one that keeps it off your back. My old TNF hydration pack also has the "Airspeed Suspension" type of mesh paneling that keeps the actual compartment off my back. I forget what the name of the TNF pack is.