Zhengyi says: Saying 'The Taming Power of the Great is firm, strong, solid, and substantial' explains the meaning of Daxu. 'Firm and strong' refers to Qian: Qian's form is firm and its nature is strong, hence 'firm and strong.'
'Solid and substantial' refers to Gen: Gen's form is still and resting, hence 'solid and substantial.'
'Radiates brilliance and daily renews its virtue' means that because of being firm, strong, solid, and substantial, it can shine brilliantly and daily enhance and renew its virtue. Without firmness and strength, it would be feeble and weak, inevitably retreating when weary.
Without solidity and substance, it would be hollow and shallow, inevitably withering after flourishing; how could it long maintain brilliance and daily renew its virtue?
Regarding the commentary 'Whenever things withdraw after being sated' to 'solely firm, strong, solid, and substantial'—Zhengyi says: 'Whenever things withdraw after being sated, it is due to weakness' explains 'firm and strong' in the classic.
If not firm and strong, one grows tired and withdraws; being firm and strong, one's endeavors daily advance without retreat. 'Whenever they wither after blossoming, it is due to shallowness' explains 'solid and substantial' in the classic.
Things that bloom temporarily only to fall immediately do so because their substance is hollow and flimsy. If one is earnest, thick, full, and substantial, one constantly preserves glory and beauty without falling. 'Firmness ascends and honors the worthy.'
Zhengyi says: 'Firmness ascends' refers to Nine at the Top. Qian's firmness moves upward, and Nine at the Top does not resist it; this is honoring the worthy.
Regarding the commentary 'This refers to Nine at the Top' to 'what is meant by honoring the worthy'—Zhengyi says: 'This refers to Nine at the Top' means that the virtue of Nine at the Top sees Qian advancing upward without resisting, which is honoring the worthy.
'Dwelling above and achieving great permeability' explains Nine at the Top's 'carrying Heaven's highway, success'—dwelling above in permeability. Residing above and responding below to Heaven, it possesses the virtue of great permeability.
'When firmness arrives, it does not resist': possessing great permeability, seeing Qian arrive without resisting is the meaning of 'honoring the worthy.' Zhengyi says: Explaining the meaning of 'furthering through correctness.'
Why Gen can stop Qian's strength is because its virtue is capable of great correctness, hence 'able to stop strength.' From 'Not eating at home brings good fortune' to 'responding to Heaven.'
Zhengyi says: 'Not eating at home brings good fortune: nurturing the worthy' explains why not letting the worthy eat at home brings good fortune.
Because those above possess the substance of Great Accumulation, they nurture these worthy people and do not leave them to eat at home.
'Furthering to cross the great river: it responds to Heaven' means that by honoring the worthy and responding to Heaven with great correctness, one can surmount perils and hardships, hence 'furthering to cross the great river.'
Regarding the commentary 'Possessing the substance of great accumulation' to 'it furthers one to cross the great river'—Zhengyi says: 'Honoring the worthy and restraining strength' means Nine at the Top does not resist the arrival of firmness, which is 'honoring the worthy.'
Gen being able to restrain firmness is 'restraining strength.' Hence the classic above says: 'Firmness ascends and honors the worthy.' Wang's commentary says: 'This refers to Nine at the Top.' It also says: 'Able to stop strength, great correctness.'
Wang's commentary says: 'Nothing is stronger than Qian, yet it is able to restrain it; without great correctness, none could achieve this.' This discusses the Gen trigram as a whole.
Clearly, 'honoring the worthy' refers to Nine at the Top, while 'restraining strength' refers to the Gen trigram.
'Great correctness responds to Heaven' refers to Gen: as stated earlier, 'able to stop strength, great correctness'—stopping strength is Gen; responding to Heaven means the upper trigram Gen responds to the lower trigram Qian, hence 'responding to Heaven.'
This takes the upper and lower trigrams as corresponding, not referring to a single yin and single yang corresponding.