The flapping head has the each blade grips spindle rod bolted onto a a flat piece of steel that bolts to the head. Each blade grip has it's own and can flex up or down individually. Unlike a regular head that the spindle is one piece through the head for both blade holders. Having a one piece spindle for both blade grip has much less flex for 3D but the flapping head is more stable for scale, it let's the blade flex up like a real helo has, kind of like dihedral on an airplane.