High resilient (HR) foam
Branded as Resteezee, this product is manufactured using HR foam. This foam was originally designed to be used in furniture for boats and cars due to its buoyant/resistant property. It has a rubber/springy feel, thus works very well as an underlying support layer to balance out the body for even weight distribution. (In effect it works as the spring coil equivalent without the worry of the springs wearing or popping out over the years).