Extensive study of ripple diffusion in strongly inward shifted LHD configurations has revealed the crucial role of the bumpy component. The bumpy component plays a role in increasing surface element around half of the toroidal period through a decrease of the magnetic field strength, which increases [decreases] local radial drift per surface element around half [the beginning and the end] of the toroidal period in such strongly inward shifted configurations. This characteristic causes the lowest ripple diffusion level around a case of Rax = 3.53 m at vacuum among a wide range of Rax, which provides the physics basis for the important finding of 'neoclassical transport optimized configuration' in Murakami et al (2002 Nucl. Fusion42 L19). Here Rax denotes the magnetic axis position.