Dear JiYuan.Men,
What is called the “axial reduction shape function” in ElastoDyn is the radial shortening of the blade, associated with transverse deflection. That is, as the blade deflects in flap or edge, each node will move axially (radially) closer to the root because the distance along the deflected (curved) blade remains constant. This axial deflection is important for centrifugal stiffening of the blade. This is discussed further in the following forum topic: Coupled blade modes in FAST.
Best regards,