It is a metaphor that things that exercise regularly are not easily eroded.
This is also a metaphor. People can keep healthy through regular exercise. The same as "Hu Shu is not embarrassed."
Things tend to move easily.
Hub: the hub of the door; Decay: Decadence, corruption.
A door shaft that rotates frequently will not rot. It is a metaphor that things that exercise regularly are not easily eroded.