Complex sentence: The ultimate expression of this focus is the ability to concentrate his body's natural energies into his hand, manifesting as a supernatural glow around his clenched fist, making his fist "like unto a thing of iron".
Split into simpler sentences: The ultimate expression of this focus is the ability to concentrate his body's natural energies into his hand, manifesting as a supernatural glow around his clenched fist. This is said to make his fist "like unto a thing of iron".