When you turn a normal truck, the hanger web puts pressure on one side of each bushing (top and bottom). This squeezes that side and pushes the other side of each bushing away from the kingpin. We call this a single action bushing because only one side of the bushing works for you at a time.
The HardCore insert in the center of BONES bushings connects the two sides of each bushing together by bonding them to the HardCore insert. When the hanger web squeezes one side of a HardCore bushing, that side of the bushing reacts just like a normal bushing, but the chemical bond between the HardCore and the cushion keep the bushing from being squeezed out the other side. It actually increases the resistance to turning because it is held in place by the HardCore insert. This means basically that BOTH sides of the BONES HardCore Bushing are resisting your turning force. That’s why we call BONES Bushings the first double action truck bushings.